Check valve is going to do nothing. If installed correctly, all that prevents is tank water from going back into mixture. It won't prevent the mixture from getting into the tank (or else the gas wouldn't be able to get there
Gotcha. That makes sense, yes. I'm still pretty confident that the contents of the bottle did not siphon into the tank somehow - given the bottle was mounted *above* the tank, and every single time I checked it there was a positive flow of CO2 into the tank, I don't see how that could have happened. Could I be missing something there?
Not sure if a "drop" would do anything, and to be honest this doesn't sound right....
"Doesn't sound right" how, exactly?
The amount that spilled out of the bottle onto the tank lid probably wouldn't have been more than around 10ml, max, at a guess. What happened was that I took the cap off and (foolishly) attempted to top up the water in the bottle; this caused the contents to foam and a bit of foam to spill up and out onto the tank. I mopped virtually all of it up off the glass lid with an absorbent cloth; a little dribbled down the outside front of the tank. I checked the edges of the lid to see if any seemed have run into the tank; I didn't see any but I mopped it with the cloth just in case. So it's possible that a small amount might have got in, but I don't think it'd be more than a few drops if any. Does that sound like it might be a possible culprit, do you think?
kH dropping off, would mean that you would have a volatile pH suceptable to big swings (which could kill fish), but a high pH is unlikely.
I'm sure you have more experience with this stuff than me (not hard!) so you're probably right. Perhaps you can explain to me exactly *why* a high pH is unlikely, under the circumstances? Given that pH, KH and CO2 are tightly coupled, if the CO2 is high for a given KH, wouldn't that lead automatically to a high pH? Or have I misunderstood how this works?
I still think that something else is going on here. This just doesn't make sense that a DIY Co2 would reek this much havoc on a tank that size (and even in that short of a period of time).
Yes, it did all seem to happen very quickly. I've been keeping aquariums for twenty-odd years, and I've never seen anything like it. But then, this is the first time I've tried a CO2 system of any kind. *shrug*
I am still suspecting tank contamintion in some way or something coincident happened.
OR... how old is the tank out of curiosity? is it cycled?
Oh, it's well and truly cycled. :-7 It's been running in its current position since 1994. One of the fish that died yesterday was a 10-year-old clown loach.
The only *new* things that could have affected the tank in the last couple of days are things related to the CO2 setup. Everything else is as it has been for quite some time. Nothing other than a splash of yeast mixture has been spilled on the tank, no new chemicals (apart from CO2-related stuff) have been introduced. The cat hasn't puked in the tank. Honestly, everything else is as per normal.