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Newbie confused by KH/pH reading vs. drop checker vs. .....?

Hannah

New Member
Joined
29 Mar 2021
Messages
6
Location
Oxfordshire
Hi everyone,

I'm trying to get my CO2 dialled in on my 310l and I'm not sure what to make of my readings - I'd welcome feedback from more experienced eyes to help me undertand what I'm looking at.

For reference, my dKH out of the tap is typically 13-15, and I'm dosing 15ml of Tropica Premium Nutrition/day. The tank is 60cm deep and moderately planted with easy plants - amazon swords, java ferns, anubias etc., with a 59w LED light.

My pH readings before (left) and 4 hrs after adding CO2 (right). I'm reading these at about pH 7.5 ish and pH 6.6 ish, so drop overall of about pH 0.9. Now I've read that the ideal drop is about 1, so in theory this should be OK and indicates a CO2 concentration of just under 30ppm? Interestingly my dKH has also dropped to 8-9dKH over the course of the day - am I right in thinking this is a result of neutralising the increasing acidity?

pH.jpg


But....

The colour of my drop checker has barely changed across the course of the day - No CO2 on the left, and 4hrs with CO2 on the right (sorry for the strange angle of the photos - I have a black background in my tank so its easist to view the colour of the DC liquid from behind). The DC is positioned on the other side of the tank from the diffuser, and the diffuser itself is situated a couple of inches below a powerhead and the filter outflow, so the bubbles are well circulated around the tank.

Drop checker.jpg


I've checked my readings with the CO2 running against the pH/KH/CO2 chart found via Google, and with a pH of 6.6 and a dKH of around 8-9, that would suggest my CO2 is somewhere around 60 - 65ppm? If this is the case wouldn't I expect to see my drop checker turning more yellowish? Also my fish are behaving normally and don't appear lethargic/distressed/gasping for air.

Table.jpg


So I'm a bit confused! Can anyone help me out please?
 

dw1305

Expert
UKAPS Team
Joined
7 Apr 2008
Messages
12,072
Location
nr Bath
Hi all,
So I'm a bit confused! Can anyone help me out please?
It is quite a confusing subject. I'll start but some-one, who knows what they are talking about, will probably add some content to correct any bits that aren't quite right.

I'm <"not a CO2 user">, but you <"need to use the drop checker"> colour as your indication of your CO2 level.

This is because the drop checker has both an air gap and is filled with "4dKH" solution. This means that it is only the CO2 that diffuses across the air gap, into the 4dKH solution, that changes the pH. The dKH/pH/CO2 chart only works for the drop checker, with <"bromothymol blue"> as a narrow range pH indicator.

In the tank water you have the complicating effects of the changes in <"oxygen : CO2 ratio"> etc. which will change the pH.
Interestingly my dKH has also dropped to 8-9dKH over the course of the day - am I right in thinking this is a result of neutralising the increasing acidity?
That is a bit of a strange one, the dKH test doesn't actually measure dKH, it measures alkalinity, and in normal circumstances that is a reasonable approach, but when you add CO2 you reduce the alkalinity without changing the dKH. The reason the pH falls is because you have <"more Total Inorganic Carbon (TIC)">, due to the CO2 addition.

cheers Darrel
 

Andy Pierce

Member
Joined
27 Nov 2020
Messages
126
Location
Cambridge, UK
Heh. £0.02 worth:

Get something white you can put behind the drop checker (even if temporarily) so you can read it properly. I use white credit card blanks (www.amazon.co.uk/gp/product/B01H3KVNT6) which are also great for scraping algae off acrylic tank walls without scratching. The drop checker can take a couple hours to equilibrate so to be fair if you're doing the pH/dKH readings at +4 hours you should do the drop checker read at +6 hours. The chart you found on google is based on a very commonly propogated erroneous calculation of the underlying chemistry and slightly under-represents the CO2 concentration by about 4 ppm. This is pendantic since 4 ppm either way won't matter but it still bothers me so I use a chart: Drop checker | Fireplace aquarium based on the corrected calculation CO2 = 15.65*dKH*10^(6.35-pH) at 25C.

The drop checker can be very dependent on where it is located in the tank. I'm sure @ceg4048 can elaborate upon the importance of good flow to properly distribute CO2 through the water column - good flow will also significantly reduce the geographic dependence of the drop checker. You might think you have proper flow... but do you really? ;) Try relocating the drop checker - if you get a different answer in a different location you have flow issues to sort out first.

Have some patience and enjoy the ride. It's hard to square the "I'm measuring this right now so I want my answer right now" with the slow-motion nature of the hobby, but the real test is how well your plants (and animals) are doing for a given setup, and that takes about a month to start to get a good sense of whether things are going the right way after you make a change.
 

ceg4048

Expert/Global Moderator
UKAPS Team
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11 Jul 2007
Messages
9,310
Location
Chicago, USA
I've checked my readings with the CO2 running against the pH/KH/CO2 chart found via Google, and with a pH of 6.6 and a dKH of around 8-9, that would suggest my CO2 is somewhere around 60 - 65ppm? If this is the case wouldn't I expect to see my drop checker turning more yellowish? Also my fish are behaving normally and don't appear lethargic/distressed/gasping for air.
Yes, exactly. This is why it's not a good idea to rely on the table as it doesn't really tell you anything useful and can get you into all sorts of trouble.
As usual, we have to ask the standard question; what water are you using in the DC? If you are using tank water or tap water then the readings will be corrupted in exactly the same way that you are reading corrupted data on the chart.

It is not really necessary to move the DC around. That will not really help. Wherever the DC is mounted it should read a useful indication. The reading at the chosen location should be used as a proxy for the tank as a whole, even though we know that the CO2 concentration in the tank is never homogeneous.

I suggest also that you invest in a quality pH probe. Good probes are pricy and require constant re-calibration, but are worth it as they are less tedious to use.

It really should not require 4 hours to drop the pH by 0.9. This may indicate a fault either in your diffusion technique, flow/distribution, or injection rate - or all of these. Rather than photos of test kit readings, the answer requires photos and descriptions of the methods in which you are diffusing the gas and distributing the filter output. Important data, such as tank size, filter flow ratings and lighting specifications will also help.

Cheers,

Cheers,
 

Mark Nicholls

Member
Joined
15 Feb 2020
Messages
60
Location
Stevenage
I totally agree with ceg4048.
After spending ages looking up charts, reading articles, calculating % etc. I realised that I was spending more time researching co2 than I was looking at my tank.
I suspect that in a 310ltr aquarium, your issue is flow related. Is the filter effective at circulating the water to ALL areas of the tank or do you have dense plants or rock work that can slow it down?
I only have a 50ltr tank so flow SHOULDNT be an issue but its amazing how a decent clump of Limnophyllum will slow down flow.
There is an easy way to check water flow in an aquarium.
Gently put a few drops of food colouring into the water and watch how the coloured water moves within the tank when the filter is turned on.
 

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