A low KH makes pH more unstable. If you read my first post I did say there was too much CO2 which does poison fish. I suggested aerating the tank to counteract this.
If I added CO2 to my tank with a KH of 6.0, my pH would crash, there is not enough KH to buffer the pH. That is where KH comes into this. Also the lower my pH goes consequently the KH does lower. If you have hard water you are lucky in this respect, damn lucky. I have to check the KH regularly before waterchanges and add Bicarb to the water going into the tank.
The advice Truckasaurus gave was to raise the pH with Bicarb, this would have raised the pH quickly which would have reshocked the fish. I believe this would have caused more suffering for the fish.
Lets step back a little.
You said that Co2 is lowering the Ph. Correct but you then said that the KH could've been lowered considerably. Why would it have. when does CO2 injection mean the KH could've been lowered considerably?
Truckasaurus never advised to raise Ph with bicarb!!! He advised to raise KH with bicarb!!! That would've raised the bicarb quickly yes but I don't see him suggesting to throw aload in in one go!!! Personally I think that it is a no brainer as you are then interfering with the parameters for no reason if we are going by CO2 lowering KH and fearing a Ph crash!!! These myths need to stop.
6 is not a low KH. Many many users of CO2 have much much lower than this with no intent of altering their parameters. Do they have problems? NO of course they don't
You've got this all wrong!!!
If you read my post you will see that I am saying I doubt at all wether CO2 is poisoning the fish with a yeast setup!!! It could be but not highly likely. It is very very hard to get the amount of CO2 from yeast setups that we are getting with pressurised!! I also suggested that it could well be O depletion if the plants are failing and that surface turbulence (whilst it does drive CO2 off) will bring more O in or at least replace the O that is being used.
CO2 does not directly lower KH. It does lower Ph. If you added CO2 to your tank of 6KH and have Ph crashes then there is something else at play here!!!
KH is affected only by the addition and removal of carbonate. That is why injecting CO2 does not decrease KH. Biogenic decalcification does reduce KH.
Ph and KH can and often do move together but not in this case.
If you don't believe me then check out some of the tanks by Dave Spencer who I believe has 0dKH. Look at his fish and plants.
Rather than go into this completely take a look at this thread. I could've chosen from many by several different low KH users of CO2 but this one is short. the OP in this thread is raising his/her water with buffers in the belief that their water is too low KH for CO2!!! Read the replies. this really is one of the myths that I hate the most alongside the 'anaerobic substrate with sand' theories. Many of them spread by people who have only a little understanding of the actual facts and purely spread rumours they have always been told!!!:
http
/www.ukaps.org/forum/viewtopic.php?f=51&t=2748
There are many other myths that can be gotten rid of:
Excess nutrients cause algae - Sorry they don't
Sand compacts andcauses anaerobic areas in the substrate - Only in a non planted tank but noone makes this seperation. We want anaerobic in a planted tank
4WPG is needed to grow highlight plants - No it isn't unless you are using T12s from the seventies
There are many many more but this Ph/KH/CO2 one is spread around as if it is gospel. It is wrong and should be trodden into the carpet for good!!!!
AC
your not going to poison fish with DIY CO2
