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Over thinking surface gas exchange?

idris

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I keep learning new reasons why my tank is badly designed. (I designed it!)

It is 110cm (w) x 75cm (h) x 30cm (d) with a cover glass about 1cm from the surface.
Belatedly, I've learned this design reduces potential for gas exchange at the surface. I can see how this might reduce plant growth and promote algae, though I have little sense whether this represents 1% of the problem or 20%.

Whilst I was about to remake my spraybars angled slightly down to improve circulation. I am now concerned that I will be reducing surface agitation, compounding poor gas exchange.

If I'm adding Liquid Carbon, and could add an airstone, to what extent would this counter-balance the compromised surface gas exchange.
 

erwin123

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To answer your question how big a problem is lack of surface agitation, I don't think lack of surface agitation is a huge problem though it is likely to be a problem. I'm going to confess that my low tech tank has very poor surface agitation and you can see dead leaves floating on the surface that don't move at all in parts of the tank. But somehow, the tank is surviving.

The current HOB filter published flowrate is supposedly more than 10x the size of the tank. Maybe I need the water to cascade from the HOB into the tank with more of a 'splash'.
 

Wolf6

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If you dont have surface scum and assuming your glass cover has a corner cut out for filter in/outlet cables etc, I wouldnt worry about it too much since you dont add co2 (not counting liquid co2). What problems do you currently experience?
 

Wookii

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I keep learning new reasons why my tank is badly designed. (I designed it!)

It is 110cm (w) x 75cm (h) x 30cm (d) with a cover glass about 1cm from the surface.
Belatedly, I've learned this design reduces potential for gas exchange at the surface. I can see how this might reduce plant growth and promote algae, though I have little sense whether this represents 1% of the problem or 20%.

Whilst I was about to remake my spraybars angled slightly down to improve circulation. I am now concerned that I will be reducing surface agitation, compounding poor gas exchange.

If I'm adding Liquid Carbon, and could add an airstone, to what extent would this counter-balance the compromised surface gas exchange.

Your tank is long, wide and shallow, so the ratio of surface area to volume is already higher than most tanks, giving you a good starting point for gas exchange.

I find on my low techs, getting a good level of surface agitation works best to keep a regular supply of atmospheric CO2 and O2 going into the tank. For some things its possibly more important than on a mature high tech tank, as on that you are at least injecting some CO2, and the plants are usually producing more O2.

If you are using spraybars, then angling the outflow at the surface is far and away the best idea. If you are not getting enough circulation down to the substrate and to the rear of the tank, then the answer is to increase pump power, not change the angle of the spray bar. Doing that gives with one hand, and takes away with the other.

Liquid carbon can bring its own set of problems, and I wouldn't consider it a substitute for good gas exchange. Certainly well placed airstones can work very well to improve gas exchange, so that is worth doing if you can accept the aesthetic result.
 
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dw1305

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Hi all,
Your tank is long, wide and shallow, so the ratio of surface area to volume is already higher than most tanks, giving you a good starting point for gas exchange. ...I find on my low techs, getting a good level of surface agitation works best to keep a regular supply of atmospheric CO2 and O2 going into the tank. For some things its possibly more important than on a mature high tech tank, as on that you are at least injecting some CO2, and the plants are usually producing more O2.
That is the one, basically a large gas exchange surface will cause dissolved gas levels to remain nearer their atmospheric equilibrium values.

cheers Darrel
 

Hufsa

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Your tank is long, wide and shallow
It is 110cm (w) x 75cm (h) x 30cm (d)
This seems long, narrow and deep to me, am I seeing it right?
The tank in their linked journal is indeed very deep.

I would not angle the spraybar down. Point it up to get a good surface ripple, I think good surface exchange is important for low tech, so you are definitely fighting an uphill battle with your tank dimensions. You will probably need additional pumps to get good water movement in such a deep tank, maybe look at some of the wavemakers or what they are called that can be angled freely?
 

Wookii

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This seems long, narrow and deep to me, am I seeing it right?
The tank in their linked journal is indeed very deep.

I would not angle the spraybar down. Point it up to get a good surface ripple, I think good surface exchange is important for low tech, so you are definitely fighting an uphill battle with your tank dimensions. You will probably need additional pumps to get good water movement in such a deep tank, maybe look at some of the wavemakers or what they are called that can be angled freely?

Yes, you're right @Hufsa - I completely misread it as I'm so used to seeing the dimensions listed as 'length x width x (vertical) depth', plus the "30cm (d)" threw me off :p

So the tank actually has the complete opposite problem, a much lower than usual surface area to volume ratio, and a very deep tank to get the 'gas refreshed' surface water down to the bottom of.
 

dw1305

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Hi all,
If I'm adding Liquid Carbon, and could add an airstone, to what extent would this counter-balance the compromised surface gas exchange.
Yes, it would help. Have a look at <"aeration and dissolved oxygen">, I wrote it a long time ago (primarily for rheophilic plec keepers), but it is relevant to all tanks.
Take a ticket and join the queue Darrel, its a long line!
No, more "mister nice guy" for me, it is blame culture from now on. If it is good enough for both the Government and my employer it is good enough for me.

cheers Darrel
 

Wolf6

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Take a ticket and join the queue Darrel, its a long line! 😜
Yeah thats right, I totally didnt misread it either and you were first ;) To be fair and in our defense those are very odd dimensions and I would recommend an airstone or other way to get some surface movement realising this :)
 

Wookii

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To be fair, if I had such a deep tank, I'd be tempted to part fill it, and have a nice terrarium moss/plant wall on the rear for the rest of the height (mainly because I'd like to do something similar myself anyway).
 

idris

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Everyone can join the back of the queue on blame. The responsibility is all mine for the dimensional confusion. I hoped w/h/d would be clear enough (not least as it was consistent with x/y/z axes) but was aware of the ambiguity of the d. I wrestled with the idea of w/h/f2b but that seemed clumsy. Perhaps I should have gone with l/h/w. Oh well, what's done is done. I shall beat myself with a knotted rope this evening.
Anyway ... back to the plot ... loosely ...

@Wolf6 The gaps around my inlet/outlet pipes are pretty well sealed, as if they aren't, there seems to be enough evaporation that the marine-ply hood on the tank (which is integral to the light mounting) starts to warp ... despite around 6 or 7 coats of "waterproof" varnish.
The biggest problem currently is algae. I'm pretty sure that is at least in part due to circulation ... hence remaking the spraybars.

@Wookii My concern with angling the spraybars horizontally (as they currently are) is that there are areas at the bottom of the tank that don't get a lot of flow. I'm looking at increasing flow rate with better spraybar design (fewer, smaller holes): in such a tall, narrow tank, even a spray at 45° still only represents a drop of 30% of the tanks height (where there are negligible plants) and might help with downward momentum. (Obviously fluid mechanics is not my area of expertise!)
Meanwhile the volume of "fresh" air at the surface is only about 3 litres, and only refreshed a couple of times a day at feeding time. I have no idea of the rate at which gaseous exchange takes place, so I don't know whether that's a limiting factor.
As for a half filled tank - it seemed to work as an ecosystem for quite a few years, and I like it as it is. But having had significant algae problems recently, I'm just trying to get back to where I was ... and reading myself down so many rabbit holes (flow / filtration / light / dissolved O₂ / etc) in the process.

@Hufsa I have run two canisters (with seperate spraybars) for a long time, but as part of addressing flow I have removed media from one, so it effectively acts as just a circulation pump. When I re-plumb the tank, hopefully the flow will improve further.

@dw1305 An interesting article on aeration. (I even understood some of it. :) ) Sadly, increased knowledge rarely seems to lead me to happiness. Now I learn that ...
As I live in a hard water area, the oxygen content of my tap water is likely to be lower. Such a pity I've been trying to do more water changes recently and run the rain-water butt dry.
On the upside, my heater died the other day, so my tank is around 4°C cooler till payday.

For "airstone" location - all my hardware is under the tank, and I've got enough junk in the shed (and misguided DIY enthusiasm) to build an inline O₂ reactor (possibly a tubular EPDM diffuser in a 2" PVC tube.) I figure that's going to give a reasonable distribution, especially (if I understand @dw1305 's article) if I site that before the canister filter, thus maximising the "residence time" and benefiting the nitrifying bacteria. (I'm hoping that wouldn't lead to a problem with airlocks in the filter?)
 

dw1305

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Hi all,
if I site that before the canister filter, thus maximising the "residence time" and benefiting the nitrifying bacteria. (I'm hoping that wouldn't lead to a problem with airlocks in the filter?)
That was the major criticism (and there were a number of other criticisms) of <"Aeration and dissolved oxygen"> when the article was first "published".

I hadn't been a canister filter user for very long, at that time, and it had never occurred to me that people <"wouldn't have a pre-filter sponge"> on the filter intake scaffold.

An <"intake pre-filter"> is just a no brainer for me.

cheers Darrel
 

idris

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Fair enough. Certainly what I've read since has suggested air locks are a potential issue, so perhaps something like a DIY CO₂ reactor (only fed with O₂) on the output of the flow pump makes sense?

As discussed elswhere, for better of for worse, all my filter media is under the tank.
 

Kevin Eades

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Fair enough. Certainly what I've read since has suggested air locks are a potential issue, so perhaps something like a DIY CO₂ reactor (only fed with O₂) on the output of the flow pump makes sense?

As discussed elswhere, for better of for worse, all my filter media is under the tank.
I think the problem you will find is you will be pumping in air and not oxygen in pure form. There for 70% of what you will be adding to the reactor will be nitrogen which i belive from my limited knowledge is very difficult to dissolve into the water so you get a build up of gas. An inline co2 diffuser with an air pump on may be more effective as the gas can escape in the tank.
 

idris

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Fair point - I'd not thought that bit through.
Not being a CO₂ buff, I might be confusing reactors and diffusers :rolleyes: but yeah, I was thnking of something inline.
 
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