pH fluctuations due to CO2?

[NewbieFish]

So I just started injecting CO2 to my tank. My water has a KH of around 2.5dh. Prior to CO2 I was getting a pH of around 8. If you use that conversion chart it means I had less than 1 ppm CO2. Now with CO2 my pH is 7 which means 7.5ppm CO2. This is fine right now since my lighting isnt ready yet anyway to need higher CO2. However, I was concerned that once my CO2 turns off at night my pH probably goes back up to 8. Can the fish handle this shift in day vs nightime pH on a daily basis? When I have my lighting setup I will probably to striving for 20+ ppm CO2. That means my daytime pH will be even lower at around 6.5 or less. The water where I am is extremely soft. The stuff that comes out of the tap has less than 0.5dH KH. (I have crappy gravel which is probably raising my KH a bit). Should I be concerned?

[phanmc]

A shift in pH due to CO2 isn't harmful to fishes. My tank water sees a rise from 6.4 to over 7.5 after water changes with no ill effects to the fishes.

Most of my tanks are running with around 30ppm of CO2. Some of my tanks uses ADA Aquasoil which drops the pH down to below 6.4. I now use a solenoid on all of my tanks to shut off CO2 at night and I haven't had any problems.

A shift in pH isn't what's killing fishes in most cases. It's most likely either too much CO2, not enough O2, or a large change in kH and/or gH. The first two are easy enough to address and the latter is rarely a problem on an established tank ujnless you change your water source. To prevent CO2 overdose, use a drop checker so you know you're not going overboard. To prevent low O2, make sure you have some surface agitation for gas exchange.

[WaywardSon]

Mine goes from 7.2 down to 6.4 during the day (Co2 injection) - the fish are fine. Unless you closely control your pH with a controller that turns your Co2 on and off as necessary (usually overkill), the daily variance seems to be unavoidable but not at all harmful.

[Mgamer20o0]

i leave the co2 on all the time. there going to be ph swing from co2 but havent seen any problems yet.

[KarlTh]

You're about to be initiated into one of the Deep Magics of fishkeeping. It is this:

Contrary to popular belief, pH swings do not harm fish.

Swings in TDS (total dissolved solids) harm fish because they mess with osmoregulation. Swings in temperature harm fish because they mess with their enzymes*. Swings in pH (in a wide range from say 5 to 9) don't harm fish because they don't mess with anything.

*cold blooded animals have much larger genomes than warm blooded ones. This is because the warm blooded ones only need one enzyme to do a job - one which will work at 37C or whatever the animal's normal temperature is. Cold blooded animals need an enzyme for each narrow temperature band within the range that the animal has to function at - say one which works at 12-15C, one which works at 14-18C, another at 19-22C and so on. This is what governs the temperatures at which a fish can live - what temperatures it has functioning enzymes available for.

[jmhart]

The KH/ph chart isn't really accurate at predicting dissolved co2 levels.

Tom does a good job explaining it here and why you should have a drop checker:

http://www.barrreport.com/estimative-index/2818-drop-checkers-co2-indicators-why-how.html