TDS and the relationship within a fishtank

chris1004

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Hi, this is a bit of a spin off from another thread that is going on at the moment with regard to a rising KH due to (I think) my gravel and how to check it posted in the 'substrate' area. However I thought it better to raise this question in this area (water chemistry) as the subject of TDS arose, which is more akin to 'water chemistry' than 'substrate' and I was hoping for some enlightenment as to what it is all about. I anticipate that it could get quite interesting hense posting it here at the risk of dis-jointing the original thread under the 'substrate' catagory heading.

Anyway I use a TDS meter to check my RO water but it seems that its uses are not limited to that. I am all ears.... :)
 

Ed Seeley

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TDS measures much more than the hardness of the water. TDS means Total Dissolved Solids but actually this is a misnomer as it measures everything that is dissolved in the water, including some gases.

The TDS will rise rapidly when adding fertilisers, though the water may well be very soft. My tank uses pure RO with a bit of GH added via Kent's RO Right and Seachem's Equilibrium but the TDS last time I measured it was over 300! My koi pond with tap water but no additives added usually has a TDS of around 160 (the tap water is around 130ppm).

TDS also rises when water changes aren't kept up and the differential between input water and the TDS level in a pond is one key indicator of the maintenance regime of a koi pond and could be a key indicator for a planted tank too, but I need to think more clearly on that first.

Tomorrow I will take some updated measurements of the TDS, KH, GH and pH in my tanks, ponds and source water and add to this.
 

chris1004

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Primarily apart from knowing that the TDS of my RO water should be below 1ppm I don't know what sort of levels are cosidered high/medium or low and what kind of levels are desirable (although I would expect them to be low). I am not getting any hangups about it just interested, but then sometimes ignorance is bliss, LOL.

I have a hand held digital meter which also measures temperature which is fairly new (aprox 9 months old), therefore I hope its safe to say that it should be reasonably accurate. Yesterday morning when I dipped it in my tank it showed a reading of 231ppm, thats after a lot of water changes with known 0ppm RO product water with added remineralising agents. In fact the last 3 water changes have been 50% weekly and prior to that I was changing 20% twice a week.

I have just measured my tapwater and it is reading 372ppm. Whilst I was doing this a thought occured to me that I have no idea whatsoever what that 372ppm consists of. Whereas I have a fair idea of what the majority of the 231ppm of TDS thats in my tank consists of (I think), as I have added ferts and reminaralising agents and I know that there is organic waste there from both the plants and the fish. If as you say Ed some Gases can effect it aswell then possibly some of the 231ppm could be disolved co2, (the reading was taken in the morning before the co2 cycle started, which leads to another question does it fluctuate a lot during the day? This I will test for myself). So unless you know what the 'disolved sediment' actually is, is the reading relevant at all?

Clive mentioned 'conductivity' I know he knows what he's talking about but I have no idea what this means outside of the field of electricity and I know you can't mix that and water!!!!!! So it is leaving me somewhat perplexed.
 

Ed Seeley

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Effectively the conductivity and TDS are the same for us as the way your meter works is to measure the electrical conductivity of the water to work out the number of dissolved ions in the water and then use that to give a TDS level. This is actually only accurate to within 10%.

As to what is a 'good' level, it's very hard to say. The last time I paid a lot of attention to it in my planted tanks was when I was running a 12" cube and keeping Chromaphyosemion bivittatum 'Funge' in there. I had a pair from a breeder that simply refused to lay eggs despite the fact that they had been spawning for him! I was using pure RO with a little tap water added so tested all the water parameters to see what was going on and found a huge TDS differential between the new RO water going in and the TDS level of the tank. This was due to the fertilisers going into the tank. I cut back the ferts (reducing the TDS) and the fish started breeding again.

Interestingly my current main tank has more Chromaphyosemion bivittatum 'Funge' in it that are breeding happily and the fry survive and grow well in there. I dose this more leanly than many but still add enough ferts. I think the big difference is these are all fish I have raised myself and they have always lived in these conditions.

In koi keeping, the TDS is something that is being realised to be key towards getting the best out of your fish. In the mud ponds in Japan the water can have a TDS of 30ppm. This is so soft that a quick rainstorm can cause problems so the breeders have to add hardness. In soft water the fish seem to grow quicker and the colours develop better and don't deteriorate. The level people tend to be recommended over here is around 80-100ppm. People mainly use a mix of RO and tap water to achieve these levels.

What is also looked at is the TDS differential. If you're adding water of say 100ppm but your pond has a TDS of 250 it means that extra 150ppm is coming from the waste from the fish or other things that are being added or the accumulation of TDS by evaporation. Basically the closer you can get the pond TDS to be compared to the new water TDS gives you a great indication of how clean your pond is.

I'm not sure what effect CO2 will have on TDS. As TDS meters measure ionic compunds dissolved in water so the proportion of CO2 that converts into carbonic acid will increase TDS measurements I imagine at least.
 

chris1004

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Thanks Ed I understand the referance to conductivity now and the context in which it is refered to. I am going to check the TDS of the fresh water that I add to my tank tommorow (RO +reminaralising agent) when I do a water change just as a matter of interest so that I have a rough idea just how the reminaralising agent is affecting the overall TDS. Is there anyway that I can get a rough idea how the ferts are affecting the overall reading? i.e What I mean is are there any tests that have been conducted where say 1ppm of a fert (k2SO4 for example) adds x ammount of TDS to your water?

I know its not necessary (even a bit 'anorac' possibly) but I do find it very interesting, not sure how it could help me with fishkeeping / tank maintenance though.

Although its fair to say that I don't know what I'm talking about here I reckon it would be far better to know what solids are dissolved within the TDS than not, which is a great advert for using 100% RO water as a platform to work from. For instance at the moment I think I would rather have a TDS of 500ppm but know that there is nothing harmfull in there than say 200ppm which has got some heavy metals, chloramines or other 'nasties' as part of its whole.
 

Ed Seeley

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You'd have to do subsequent specific tests to identify the constituents of the TDS to know what's there really.

I agree a high TDS in a planted tank isn't necessarily a bad thing, although the work being done with the effects of low TDS water on koi are making me think about that a bit more...

I'll add some results tests later too.
 

chris1004

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ok done a water change this afternoon but didn't bother taking a reading of the replacement water as I know it to be less than 1ppm as I didn't bother to add remineralising agent today in an attempt to reduce the overall KH. However I did check the tank before and after and realised somthing interesting.

First you can use the TDS meter to assertain the true volume of water in your tank. If you know how much 0ppm (call it 0 for ease even though it could never actually be) RO water is used to replenish the tank to its previous level and you know the starting and finishing TDS values. i.e

start 300ppm
finish 150ppm
100 litres replenished
total water volume 200 litres.

Secondly a start and finish TDS reading can be taken when adding ferts of any mixture, the results of which could be used to glean more info as to what is going on in your tank I'm sure, I just don't know how yet.

I understand that as there is a 10% margin of error to consider with these meters an isolated result on its own will mean little but if your were to take the average of say 10 results then it would soon become very accurate indeed, in fact the more test results you take the average of (when measuring for the same thing in the same way) the more accurate the overall result would be.

Using the above method it would be easy to determine how any individual thing that you add effects the water with regard to TDS and be pretty confident of the result.

I need to think a bit now as I don't really know how this info could benifit me yet, but I feel that it could. I'm certainly going to be playing around with my TDS meter a bit over the next week or two and trying to think things through.

Probably teaching me nan how to such eggs here LOL.
 

Ed Seeley

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chris1004 said:
I understand that as there is a 10% margin of error to consider with these meters an isolated result on its own will mean little but if your were to take the average of say 10 results then it would soon become very accurate indeed, in fact the more test results you take the average of (when measuring for the same thing in the same way) the more accurate the overall result would be.
Unfortunately not mate! That would only be partly true if it was on a range of totally independant readings with vastly differing conditions. 10 readings on the same tank would be measuring the same system with potentially the same consituents causing the error. The error here is caused as some solids in the water are not be picked up by the conductivity method used to measure them so their percentage is approximated when calculating the TDS value. If your system has lots, or none, of these then it will always throw your measurement off by a similar value. It's a measurement error, rather than an inaccuracy.

However the size of the error isn't going to be big in the values we're looking at as it will almost certainly be way less than 10%.

I'm not sure how useful TDS is going to be in a standard planted tank to be honest. For breeding soft water fish and growing koi for shows fair enough. It will let you see the effect of adding anything to the tankbut I'm not sure what you'd do with the information, if you know what I mean?
 

chris1004

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Yeah I do know what you mean, just 'playing' around a bit with the test meter thats all. I certainly wouldn't change anything due to the test results until I fully understood what they meant and I can't see that happening sometime soon.

Just a big boys toy then really :lol: :lol: :lol: .
 

chris1004

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Ed Seeley said:
Unfortunately not mate! That would only be partly true if it was on a range of totally independant readings with vastly differing conditions. 10 readings on the same tank would be measuring the same system with potentially the same consituents causing the error. The error here is caused as some solids in the water are not be picked up by the conductivity method used to measure them so their percentage is approximated when calculating the TDS value. If your system has lots, or none, of these then it will always throw your measurement off by a similar value. It's a measurement error, rather than an inaccuracy.
I've been thinking about this all week, can't seem to shift it from my mind. If the same constituants cause the same errors on each individual tank would it really matter as the measurement will be out the same amount each time. Therefore any differential reading (i.e. before and after somthing was added) would still be good surely.

Anyway I now have pretty much a weeks worth of readings which I will try to get posted up on here soon, if your still interested ED.
 

Ed Seeley

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Post away mate!!! (I've still not taken any sorry!!!)

The error in the way we measure TDS is a measurement erro, not a random sampling error. It is caused by the fact that we actually measure the conductivity, not the real Total Dissolved Solids. So everytime you measure the TDS of the water the meter measures conductivity then calculates an approximate TDS based on what an 'average' water sample's real TDS would be if it had that conductivity.

When we usually measure and take multiple samples to get a more accurate reading we are assuming that the measurements we take will be randomly distributed either side of the 'real' level so taking an average of those readings would give us a measurement closer to the real value. This is because these are assumed to be random sampling errors rather than a measurement error.

With TDS measured via conductivity in the same water sample we may have a value that is actually to one side of the true TDS due to the way the TDS is calculated. For instance if you had water that contained more than the average solids that conduct then the reading may be too high and vice versa. RO water remineralised with GH booster for instance, will have a hardness composed almost completely of salts that will conduct so the reading may be higher than the average.

For very low readings though we are probably talking one or two degrees out so it's not a big deal!
 

chris1004

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Ok, I think I understand the above but I am still stuck in the mindset that we can be certain by measuring TDS how somthing we add effects the TDS of our tanks and therefore can be determined, how that could help us I have no idea though.

If my measurement was out by 20ppm of absolute correctness for arguments sake due to the type of dissolved sediment within the tank prior to say adding a dose of ferts, then it will still be out by the same margin afterwards, therfore is it safe to assume that the differance between measurement A and measurement B is the ammout of TDS added by the ferts, I think? There may be a small discrepecy due to the ferts conductivity differential itself but it surely can't be very much?

Getting on to the TDS measurements that I have taken this week, first I have to say that this is far from a scientific study I only took measurement as and when I remembered and I got my dosing days muddled up as you will see later (its just unnatural to be awake at 6am how are you supposed to remember what day of the week it is, LOL) but a couple of interesting (?) points did emerge.

SAT 31st of jan pre water change the TDS was 256ppm and the TDS post water change carried out at around midday was 130ppm, and a PH and KH measurement of 6.6 and 4.0 respectivly was recorded.

SUN 1st feb o6oohrs a TDS of 136ppm pre macro dosing and 154ppm at 1800hrs.

MON 2nd feb 0600hrs a TDS of 157ppm pre micro dosing. At 16oohrs TDS 158ppm and a PH and KH measurement of 6.6 and 5.0 respectivly. At 2100hrs a TDS of 159ppm was measured.

TUES 3rd feb 0600hrs a TDS of 161ppm pre macro dose was recorded and 10minutes later at 0610 the TDS was 175ppm. At 2005hrs the TDS was 174ppm and at 2210hrs 173ppm.

WED 4th feb 0600hrs a TDS of 176ppm pre macro dose was recorded and at 0700hrs the TDS was 198ppm. At 1930hrs the TDS was 194ppm with a PH and KH measurement of 6.8 and 5.0 respectivly.

THUR 5th feb 0600hrs a TDS of 199ppm pre micro dosing and at 1830hrs a TDS of 197ppm.

FRI 6th feb 0600hrs a TDS of 203ppm was recorded and at 1800hrs it was 202ppm with a PH and KH measurement of 6.7 and 5.0 respectivly.

SAT 7th feb 0830hrs a TDS of 202ppm was recorded and at 1800hrs it was 199ppm with a PH and KH measurement of 6.6 and 6.0 respectivly.


Points possibly worth considering.....

1/ The KH hasn't risen as much this week as it has been doing befoe but then I didn't add any remineralising agent at all this week, could this be having an effect? (not of the starting kh but of the rising kh).

2/ Nearly every day a fall of a few ppm was seen during the photoperiod. I think that it was due to the fert uptake from the plants as could be reasonably be expected.

3/ nearly every night a rise in the TDS was recorded possibly because the plants were not uptaking any ferts and the rise was probably due to organic waste production. Interestingly it was fairly consistant, could this mean that it is consistant over 24hrs and some is added during the day aswell meaning that the daytime fall in ppm recorded is not all fert uptake but that some of the fert uptake is offset by organic waste production meaning there is more uptake of ferts than there initially seems?

4/ The micro fert dose didn't appear to add much to the overall TDS value wheras the Macro did, I suppose as you would expect.

5/ The co2 seemed to have no bearing on the expected results unless of course you think that the co2 was the reason for the fall in ppm during the day.

6/ In my case it would appear that I have an improving situation as this time last week my TDS ppm was over 20% higher.


I am going to continue monitering my tanks TDS next week but I will try to be a bit more consistent with the timing of the tests. If your still interested ED I will post on here next week.
 

Ed Seeley

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Interesting stuff Chris. You're dead right about the change in TDS measurement being a good indicator of the real change in TDS and possibly useful. I been thinking that the TDS differential between tank water and input water might be a good indicator of conditions in our tanks (once you understand the effect of ferts on the TDS) like it is in koi ponds. The ferts will just make things a little more complicated.

Your logic for the changes you've seen seem pretty solid to me. Good hypotheses IMO.
 

chris1004

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The rising KH value throws things out a little as I don't know how many ppm is added to the overall TDS per 1degree kh rise although I would expect this rise to be fairly consistent over time (which is also backed up by the kh test results). Which may be partly what I am seeing with the overnight results and is probably in addition to organic waste production during the night. Again I think it would probably continue through the daytime at the same or similar levels as I believe that the organic element would be added by the breakdown of organic matter within the filters as opposed to excrement added directly to the tank water and would therefore also be fairly consistant over time.

I think its worth reiterating that I have no idea what I'm talking about and I'm not a chemist etc. Merely trying to think things through aloud as it were.

I could possibly be considered a bit of an anorac on this one at the moment though.LOL.
 

a1Matt

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chris1004 said:
I could possibly be considered a bit of an anorac on this one at the moment though.LOL.
I think you are definitely a little bit of an anorak... can I join in too please :)

I saw this post which I thought might be of interest to you...: http://www.plantedtank.net/forums/gener ... post172828

If you have any more updates on your readings I will read them with interest.

My angle is that I am running my tank as a 'non CO2, no water change tank' so do not have the luxury of the EI style water changes to reset my fert levels and clear organic waste buildup. So I am contemplating buying a TDS meter so that I can see if 'things' are building up in my tank. Not as a necessity, more as a curiosity.
 

Ed Seeley

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Bear in mind Matt that if you're dosing with salts my experience was that they upped the TDS very quickly! I'm not saying don't use the TDS meter but don't worry if the readings are higher than you expected!
 

a1Matt

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Thanks Ed. I am a complete noobie to measuring TDS (or any measuring of the tank really), so all advice appreciated. I have got only the vaguest idea of what my TDS will be to start with (not 0 and not 50,000! lol)

I am expecting that the powder ferts I have added will probably form the most part of the TDS readings. My hopes are that I can draw some correlation between what I add in and what gets used up in the tank without the 'organics' affecting the reading too much. I don't mind that things will not be precise. Just if they are going up or down will be a good start!

I plan to start with not adding any ferts for two weeks or so (will actually add some just after the first measurement, then measure again directly after) and measure it regularly to see what happens to the TDS values in that time.

Like was said earlier by Chris... not an overly scientific methodology, just curiousity :)
 

Ed Seeley

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a1Matt said:
Thanks Ed. I am a complete noobie to measuring TDS (or any measuring of the tank really), so all advice appreciated. I have got only the vaguest idea of what my TDS will be to start with (not 0 and not 50,000! lol)

I am expecting that the powder ferts I have added will probably form the most part of the TDS readings. My hopes are that I can draw some correlation between what I add in and what gets used up in the tank without the 'organics' affecting the reading too much. I don't mind that things will not be precise. Just if they are going up or down will be a good start!

I plan to start with not adding any ferts for two weeks or so (will actually add some just after the first measurement, then measure again directly after) and measure it regularly to see what happens to the TDS values in that time.

Like was said earlier by Chris... not an overly scientific methodology, just curiousity :)
Your best bet is to measure your TDS differential at first. This means measure the difference between the water coming into your tank (Tap water I assume?) and the tank as it stands. Remember TDS is effectively everything that is in the water so it will naturally go up over time with the general processes that go in in the tank.

If you're starting with tap water then it will probably have a TDS somewhere between 100 and 400 ppm.

Like you said use it as a guide and it might give you some interesting data, but don't rely on it to tell you exactly what to do until you understand all the things that go up into the TDS. And when you do understand that then come and tell us all! :lol:
 

a1Matt

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My TDS meter was ordered at 10:30pm last night and I got an email at 06:30 this morning saying it had been dispatched!

I'll start off by measuring the tap water when it turns up :) Then proceed to dunk it in every liquid in the house probably :lol:

I have seen web pages on people taking TDS readings of their tea and coffee apparently low TDS water makes better coffee, higher TDS for a cup of tea - amazing what obscurities google searching can find!
 

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