here's the specifics on both incidents...
first off, both tanks are 30 g, 7.0 ph water changes (5 g each time) averaging once a month, under gravel filters and aquamaster 250s used on both tanks
first incident happened about 6 months ago... (this is the one that might have been attributed to smoking) it was a tank stocked with 6 green tiger barbs, 2 golden barbs, 3 cherry barbs, 2 rosy barbs and one pleco... the tank basically "crashed" in that all the fish started acting ill and died one after the other... I had the tank set up for about 6 months at this point.... after I'd lost enough fish (5 or so), i took the remaining ones to the lfs, where i was told that they had been poisoned somehow... the only logical explaination was high levels of cigarette smoke in the area... i live in a "band house" (there's members of a couple bands here, so it's a high traffic house) where there are three heavy smokers, a couple of which were unemployed at the time, so they were smoking indoors a lot, so that was the best explaination we could come up with... out of all those fish, one gold barb survived and is living with all new tank mates in the same tank (after thorough cleaning and a total smoking ban in my house) and they are all doing well together
second one was in a separate tank which contained 2 glass cats, 2 glo lites, two pygmy frogs, 2 emperor tetras, 5 hatchets (2 marble, 3 silver), and a cleaner (kind of ray looking, i don't remember the species)... i left town and had my roommate feed my fish for the weekend... he last fed them sunday night around 9 pm... i got in around midnight and 5 fish were dead with a bunch of food in the tank... i did a ph and ammonia test, with results of 6.8 ph and 0 ammonia... i did an immediate 5 g water change and hoped they would right themselves, but both the cats were dead in the a.m., so i move the rest to a sparsely populated 20 g in the same room... the last hatchet died day before yesterday, and they seemed to have some burns in their gills and maybe eyes.. the only survivors are the frogs and the cleaner... most of these fish i'd had for two years, and they'd been through a lot (ph spikes, algae blooms) so i figured they were pretty hearty... i initially thought over-feeding, but my roommate swears he fed them a normal amount, but I suspect he might have been drunk and blew it... the only other thing he thought it might be was something from work on his hands like cleaning solution or something like that, which begs the question, why wouldn't you wash your hands after handling stuff like that? but it his life, but unfortunately my fishes lives too... anyways, any old pros out there got a horror story or two like this? let me know, and if you made it to the end of this post, thanks!