Thanks Bert, just about to have a read of that.
That's kinda my point though Squawkbert. I've heard about it (and heard the tales about it), but never experienced it. And all my tanks, including a 20L one have 0dKH. Some are way over-stocked (according to the guidelines!), but I've got Nitrate below 12.5 and every other parameter 0 or where it should be! Three have CO2, one pressurized, one DIY and one tablets. None of these seem to have wild pH fluctuations though and the fish are healthy and breeding.
The killi keeper who was the source of this had a small tank 12"x8"x8" with low load (6 small killifish) and a few plants. No CO2 and a crash.
You're dead right about a CO2 dump killing stuff, but I think that's to do with Co2 excess and O2 shortage killing the fish not a pH crash. That shouldn't have happened to the killi tank.
Thanks for the post though, I do appreciate everyone's contribution. It's good to hear your views.
Does anyone out there use very soft water like me without problems???