# Rainwater & CO2



## maj74 (7 Feb 2021)

Am I right in thinking that I have to be a little more careful with CO2 in my tank given I am using rainwater?

Presumably the low hardness means that co2 stopping and starting can cause larger ph changes?

And the low hardness in rainwater makes the ph swing for the ideal CO2 concentration larger?


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## ceg4048 (7 Feb 2021)

maj74 said:


> Am I right in thinking that I have to be a little more careful with CO2 in my tank given I am using rainwater?
> 
> Presumably the low hardness means that co2 stopping and starting can cause larger ph changes?
> 
> And the low hardness in rainwater makes the ph swing for the ideal CO2 concentration larger?


Hello,
         Its unclear whether you are using a pH controller or a simple uncontrolled injection, however we have to exercise caution when using CO2 regardless of whether rainwater or tap water is being used.
CO2 dissolves in tap water just as easily as in rainwater so the injection rate will be the same, however, as you mention, the pH excursions will differ between the two. This is one reason using a dropchecker can be useful in stabilizing the injection rate.
The important thing to remember is to not put the cart before the horse. CO2 kills fish but fish do not care about pH or pH swings. 

The goal when using CO2 is to dissolve enough into the water to satisfy plants needs but not so much that the fish are poisoned.
That amount is the same regardless of the type of water.

The best thing to do is to first aim for a 1 pH unit drop from the natural pH of the water and monitor using the DC. If the DC shows only a dark green, for example, then this tells you that you can inject at a higher rate until the DC shows a light lime green. This will then tell you how much of a swing from it's natural pH that your rainwater will have.

Cheers,


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## hypnogogia (7 Feb 2021)

Do you use pure rainwater, or do you cut it with tap?  It all depends on the Kh of your water.


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## jaypeecee (7 Feb 2021)

Hi @maj74 



hypnogogia said:


> It all depends on the Kh of your water.



As @hypnogogia says, it depends on the KH of the water - whichever way you make it up. KH is often known as carbonate hardness but this isn't strictly correct. It is more accurate to refer to KH as alkalinity. Rain water itself is likely to have a KH of around 1dKH. That provides negligible buffering against pH fluctuations from injecting CO2 (or other aquarium processes). The other thing to consider is GH, general hardness. Both KH and GH can be adjusted independently of each other using household compounds.

JPC


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## alto (8 Feb 2021)

ceg4048 said:


> fish do not care about pH or pH swings.


*Tolerance to temperature, pH, ammonia and nitrite in cardinal tetra, Paracheirodon axelrodi, an amazonian ornamental fish*
This study was done with wild cardinal tetras which as might be surmised are quite tolerant of low pH


			Tolerance to temperature, pH, ammonia and nitrite in cardinal tetra, Paracheirodon axelrodi, an amazonian ornamental fish
		


In terms of anecdotal examples, there are many - though I was once also very sceptical


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## Oldguy (8 Feb 2021)

alto said:


> This study was done with wild cardinal tetras


Thanks @alto a very interesting read.


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## jaypeecee (8 Feb 2021)

Hi @alto 

Thanks a bunch for that priceless gem of a document! For me, the take-home message is:

"Lethal ammonia concentration (LC50) for cardinal tetra was calculated to be 23.7 mg/L NH3+ NH4+ or 0.36 mg/L NH3".

FWIW, the figure for free ammonia (NH3) is consistent with guidance given by _Seachem_ which, rightly or wrongly, had become my de facto reference.

Thanks again.

JPC


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## dw1305 (8 Feb 2021)

Hi all, 


jaypeecee said:


> Lethal ammonia concentration (LC50) for cardinal tetra was calculated to be 23.7 mg/L NH3+ NH4+ or 0.36 mg/L NH3"


They had really acid water, so nearly all of the ammonia/ammonium is ionized NH4+. You would only need a small addition of oxygen (or slightly warmer temperature) to kill all the fish. 

Temperature is quite an interesting one, because they are very thermophilic fish, which means lowering the temperature (reducing both fish metabolic activity and NH3 toxicity) isn't an option.

cheers Darrel


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## jaypeecee (8 Feb 2021)

dw1305 said:


> They had really acid water, so nearly all of the ammonia/ammonium is ionized NH4+. You would only need a small addition of oxygen (or slightly warmer temperature) to kill all the fish.


Hi Darrel (@dw1305),

Yes, down to pH of 2.6! But that sort of figure, I believe, is not uncommon in blackwater biomes. I seem to recall having read that it's the high level of humics that protect the fish against such low pH figures. I mentioned Seachem's guidance on free ammonia (NH3). Their figures are:

< 0.02 ppm   safe
   0.05 ppm   alert
   0.2   ppm   alarm
   0.5   ppm   toxic

Why Seachem you may ask? It's because I sometimes use their _Ammonia Alert_ to avoid having to keep doing ammonia tests (such as when checking out urea!).

JPC


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