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Co2 help required!

Jason Blake

1 Sep 2014

I seem to be having trouble with my co2 system. All of a sudden I seem unable to get my drop checker to change colour.

Prior to going into hospital I was able to get the drop checker to change at a drop of a hat and really easily. Unfortunately I had to spend sometime in hospital and the tank was looked after by my friend but was in a terrible state when i got back home. Somehow the co2 stopped working and the tank was over run with brown algae.

So now the tank is clean and is showing definite signs of healing. The plants look a lot healthier and the algae has subsided. However I can no longer get my drop checker to change colour. I have changed the co2 cylinder thinking the old one was running out. Have replaced the atomizer all the co2 tubing. Even the filtration tubing thinking that the kink in the pipe was somehow interupting the infusion. I have replaced the drop checker solution several times to no avail.

The most I can get in the colour to change from dark blue to a very dark green, whereas before I could get everything from dark blue to light yellow.

That being said there is a definite change in pH levels during co2 infusion. Pre co2 the pH is 6.8 before this sudden colour problem it would drop to 6.4 to 6.3 and the colour would be a limeade green. Now the pH drops to 5.6 and the colour is only a very dark green. The only reason the pH drops that low is because I am over reacting to the drop checker and pumping up the co2 trying to get it to change. I would adjust the co2 rate and wait two hours to see the difference. I no longer allow the pH to drop that low. The plants are still Pearling but nowhere near as much as they used to. You can also see a fine mist in the water during infusion so the co2 is definitely getting into the tank.

It's a 50 litre tank. Eheim pro 4e filter. Up inline atomizer. Co2 art dual stage regulator and solenoid. I use ro water remineralised with shrimp king gh/kh +. The substrate is shrimp king active soil.

Temp 24.2c
pH 6.8 to 6.3
Kh 1.7 dkh
GH 6 dgh

There is no wood of any kind in the tank only dragon stone.

Any help to this baffling situation would be appreciated.


Last edited:
Hi stu,

Yeah I have changed the drop checker fluid several times. The bottle of solution itself was only opened about 3 months ago when the tank was started.
The bottle of solution itself was only opened about 3 months ago when the tank was started.
this should still be fine BUT I'd try replacing it ... not much else really makes sense - if the pH drop is to be believed, there is CO2

The plants are still Pearling but nowhere near as much as they used to

OTOH if the plants are pearling less - is this reduced CO2 - or just plant return to health ???
or perhaps change in lighting?
I will replace the bottle. I have replaced everything else so I guess that makes sense.

The lighting period type and intensity haven't changed. It's a zed light led light that was bought brand new three months ago. The light period is 8 hours co2 on 2 hours before lights on and off two hours before lights off.

I don't know if any other factors would affect the lighting?
That's barely the beginning of LED lifetime - though you might check for dark LED's (unlikely)
Hi Jason,

According to your data, and using this:


Your CO2 levels are oscillating between 9.5 and 30.1 ppm of CO2, and that corresponds to a jump of color between green and yellow-green, so it should have such color. You say you changed the solution a few times and the same problem persists. Reached that point, there are no many options:

1. Liquid is wrong and you need to get a new indicator solution.
2. Drop checker has been filled with a mix of aquarium water and indicator, and aquarium water had significant amounts of ammonium (unlikely).
3. Indicator has a carbonate hardness above 4dkH (possible).

For me, the likely problem is the indicator solution (the only one makes some sense). For instance, most commercial drop checkers are using a solution of indicator with a fixed 4dkH. The reason why is, because if you look at the chart I just put, this makes the indicator to get a bright green when the levels of CO2 are OK (so, with pHs of 6.5 to 6.7 in the drop checker).

You have provided data that shows the dkH and pH are corresponding to reasonable CO2 levesl at the end of the day (a pH of 6.2 with a dkH of 2 gives you 30.1 ppm, that is OK), buty your indicator does not show up the expected color.

The simplest answer is that indicator solution is no longer 4 dkH but a higher level. Note that color of indicator is related only to the pH of the solution. The levels of CO2 then are depending on the kH and the pH that you have inside. By your numbers, as commented, the color,, if solution is OK in terms of dkH, should be the expected green or yellowish green. But...what if the dkH of the solution is not equal to 4? Then some given levels of CO2 will correspond to a higher pH in the solution.

For instance, let's assume your tank has the so-mentioned 30.1 ppm of CO2, obtained by direct measurement of both pH and kH from the water of the tank. CO2 levels get in equlibrium with indicator solution in the drop checker (usually after 4 hours of turning on CO2), so, let's do the opposite exercise. Considering the colour of the indicator, what hardness will provide such CO2 concentration? Well, assuming your colour (these can vary in intensity depending on the indicator concentration in the solution) gets closer to a pH similar to 7.4, then your kH in the indicator solution is over 20 dkH, which explains that does not matter too much how much CO2 you pump inside, the colour of the drop checker practically does not change. How can this happen? Only answer I have is evaporation of the liquid, if the bottle was not well tapped for any reason (which would cause an increment of dkH in the solution), or there are deposited carbonates inside the drop checker for any reason (for example, excess of evaporation of the tank). These carbonates would have the same effect when the drop checker is refilled.

Then, I will do two things:

a. Change the indicator solution. I mean, get a new bottle. This is cheap, so you should have not much problem with it.
b. Clean very well the drop checker. I would recommend you even use some weak acid to fill the drop checker and remove the carbonates (if acces to the inner is difficult), do this a few times, and then wash it a few times with RO/distilled water.

There is no other explanation. The Chemistry and way of working of the drop checker is very simple and "cannot" fail. The only thing that can happen is something interfering the chemical behaviour of the substances involved in the change of colour. Both actions should fix it.

Hope this help.