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CO2 injection in very soft water


27 Mar 2013
Manchester, United Kingdom
Hi All

I have very soft water with very little buffering capactiy.

I have tested GH and KH

GH is 120ppm
KH is between 10ppm and 20ppm, or about 1dKH
PH of tap water is 7 out of the tap.

I have been running a PH profile today. I started at 6.4 at 15:00, it is now at 6.1.

I daren't go much lower. Am I right in saying that with my hardness I am aiming for a PH of about 6?

Will see what colours my drop checkers are when they catch up and the lights come on.

How would you guys work with this parameters?

I have very soft water and used to add sodium bicarbonate to raise the Kh but I have also used pottasium carbonate as well which doubles as a fertiliser. Right now I have a part of the substrate as sand i collected from the beach and the shell content seems to buffer the water well enough without me adding anything else.

I think there is something approaching a consensus on the idea that around 6.5 during the photoperiod is an ideal ph for most of the plants we grow, which, off the top of my head, means aiming for a kh of about 4. So you could just try with regular bicarbonate of soda used for cooking rather than getting into the realms of ph in the 5's.
I wouldn't mind the low pH, I've had it in my tank from 6.5 down to 5.3-5.4. Fish didn't matter as it is a "weak acid reaction". I nature fish can live in pH below 5, in soft water
The pH drop will be bigger in soft water, i would still aim for at least 1 point drop, maybe even bigger (1.5), just take it gradually and lower it on days when you can observe the fish.

Plantwise you are probably better of with some increased KH, just for the fish it isn't needed.
The PH is now at 5.9. FIsh are not showing any distress, but that is only .4PH drop. The drop checkers are yellow.

I guess I will have to add buffering capacity to the water and raise the KH.

Might explain why I have struggled with CO2.
Hi, as said you will likely be looking at at least a 1, likely more ph drop in low kh water.
What injection method are you using as in tank or inline diffusers can produce misting within the tank which can have a greater effect upon the drop checker readings providing a false representation of gas levels.
I have very low kh from the tap also and I don't add any buffering products to alter this. I add some Gh booster to ensure adequate calcium and magnesium.
Your plant health and livestock are the best indicators of c02, but sometimes it's about distribution within the tank also. The c02 levels will not be effected by kh, you will still need to inject the same amount to reach your desired levels regardless just the ph drop will be greater in lightly buffered water.
Hey Ady

I am using an inline atomiser. Lots of mist.

Not really too sure how to proceed. A 1 point drop would mean dropping PH to 5.4 which seems problematic
A lot really depends on your tank, plant species lighting etc as to how much c02 you will need to inject.
Easy plants with low lighting for example will do fine at lower c02 levels but if your running high light with demanding species then you will need c02 to be high and very well distributed.
I would suggest increasing injection rate slowly over the course of a few days when you are there to monitor livestock to enable you to reach a maximum level and then knock it back a little for the comfort of the fish/shrimp.
How are your plants doing? They really are the best indicator.
Hi all,
If water is below KH 4 it seems impossible to get a good pH reading
It is difficult. You can use a solid state ISFET pH meter and probe, but they are expensive.

As you move towards pure H2O (rather than the dilute solution of salts that we call "water"), pH becomes a less and less meaningful measure. The problem is that pH is a ratio, and it doesn't tell us anything about the amount of H+ ion donors (acids) and H+ ion acceptors (bases) , just their ratio.
I wouldn't mind the low pH, I've had it in my tank from 6.5 down to 5.3-5.4. Fish didn't matter as it is a "weak acid reaction".
I believe this as well, it is changes in chemistry that effects fish, not changes in pH.

If you have carbonate buffered water (high dKH) it needs a large addition of acids to change pH, but in soft, vegetated water you will have a natural diurnal cycle of pH which might go from pH5 at night to pH8 during the day, purely caused by changes in dissolved oxygen and CO2 levels.


From from Rocha, RRA.; Thomaz, SM.*; Carvalho, P. & Gomes, LC. (2007)
Modeling chlorophyll-α and dissolved oxygen concentration in tropical floodplain lakes (Paraná River, Brazil) Brazilian Journal of Biology 69:2

cheers Darrel
Hi all,
I use CaCO3 to give me a higher KH
It is a cheap, safe option. "Oyster shell chick grit" is a possibility.

Another option would be potassium bicarbonate (KHCO3), you can buy it for wine making etc.

cheers Darrel
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How would you guys work with this parameters?
What are your goals in terms of plants & growth?

My tap is KH 0-1, GH 0-2
Water board adjusts pH to ~7, but my tanks run ~ pH 6 - 6.5

My goal is a tank that grows slow/steady - eg, I planted Tropica's 1-2- Grow Micranthemum 'Monte Carlo' 10days ago, no melt & at this rate of growth, should have a thin green "cover" in a month but another month before there's any density to the carpet (I pushed very small portions fairly deep into substrate, using a single pot to cover an L shaped area ~30 cm x 50cm & 10 cm wide ie a lot more brown than green to start ...)

I use a single Tropica diffuser on a 90cm x 45cm x 52H cm tank (filter is Eheim pro 3 250) & have no problems gassing my fish :oops: - fortunately no losses!
I don't use a pH probe (I wasn't about to start compromising my dead simple water changes) or drop checker (did buy one) but just monitor fish/shrimp/plants ...

As KH is so low & tanks receive significant ambient light, I choose to run CO2 24/7, "lights off" rate is low & increases when "lights on".

I recently rescaped this tank & layered Tropica Growth Substrate, thin layer of Carib Sea aragonite sand, Tropica Aqua Soil - no idea how this will work but I'd tried using Seachem Equilibrium (likely better suited to your purposes than the alkaline buffer) ... couldn't quite dissolve the powder before adding to tank, so just dumped it in, then watched in surprise as an Oto gobbled up the white bits, then sadness as Oto was clearly uncomfortable later & DOA in the morning.
Generally fish will just taste & spit this sort of stuff out, no idea if this behavior is typical of otos (I syphoned out any solids I could find after seeing this behavior); next I tried Seachem's Aquavitro line carbonate which I like but its rather expensive when doing frequent water changes ...

If you're doing EI dosing, you can likely just incorporate potassium bicarbonate into the scheme.
(I'm using Tropica fertilizers at this time)

Shrimp appear to have no problems moulting/breeding despite the very soft water but I do feed frozen brine shrimp, bloodworms, daphnia.
Thanks everyone!

I am trying to get a fairly decent rate of growth (but not super fast so I have the lights at 40%)

I am dosing EI - double dose at the moment as I was getting a slick on the surface which seems mostly cause by lack of ferts.

CO2 is currently coming on at 3pm and goes off at 10pm.

Drop checkers go yellow but fish and shrimps seem fine.
Macek that's a good example of plant adaptation. Or maybe your measurements are faulty? ;) [ kidding]