It's a 6 month old tank... was cycling really nicely up until this episode.
We're on rain-water, usually about 6.6 but recently has been about 6.4 for some reason. I use pH up to increase the pH, no drift-wood in the tank.
I found a dead glow-light which was hidden inside an ornament, I'd say it had been there for a week or so (got so many tetras it's really hard to keep track of them all!). I got it out, but it was already decaying. I'd say that's what was causing the pH to drop?
I did quite a large water change (about 30%) and vacuumed up as much of the dead fish as possible. The next day I changed the filter (was due to change anyway). Ammonia and Nitrites are both fine, and today the pH seems to be ok, however....
I've just found one of my Golden Algae Eaters vertical and with a dark green stomach (and obviously dead) in pretty much the same place in the tank. I've whisked him out.
Is it likely my tank has some nasty disease? I haven't added any fish etc. for quite some time, but maybe the dead glow-light caused the algae eater to get sick? One of the other algae eaters seems ok, but the third one is hiding in the corner and not moving much. I hope they're ok