Two disasters in the same tank.
In my first aquarium bigger than 10 gallons, I reached too far too fast. I dreamed of a South American river tank for discus and dwarf rams. The only problem is that Oklahoma City water is so hard that it's like gravel running through the pipes. I was only paying attention to pH at the time, however. I read in my Cichlid book that Discus (Disci?

) like a pH of 6.0 or lower. The pH in the tank was sitting at about 7.6. SeaChem Acid buffer was barely moving the pH and I didn't yet know how patient I needed to be, so I resorted to "pH Down" (some off-brand that I have since repressed from my memory) It was sulphuric acid in a dropper bottle. I began dosing the tank at 24 hour intervals and nothing was happening, so I stepped up the schedule a little bit. I think I had the biochemical equivalent of a Chernobyl incident (system seeming stable against big adjustments until... WHAM!). In the space of about 5 minutes, the water in the tank (55 gallons) seemed to change to milk :sick: Total fish mortality :sad except for one (apparently mutant) corydoras. Total tank tear down, gravel washing, filter scrubbing etc. The only silver lining was that this disaster solved my first problem with the same tank...
I had purchased several small meek cichlids (oxymoron) from my LFS. Two of them were the real thing, but one was an interloper, the juvenile form of something more like a piranha

I was keeping schools of cardinals and neons in the tank, which kept getting smaller as the "small meek cichlid" kept getting bigger. I must have fed him two dozen very colorful meals before I put two and two together. I stopped buying the little tetras and he stopped growing so fast (the poor undernourished thing!) I didn't want to just execute him (her?) though, so he stayed in the tank (with other fish too big to eat) while I decided what to do. I figured he could stand to stay in the tank to keep the biofilter active while I brought the pH down for discus, and that is where this long story started.
Thanks for your patience, I hope that the (horrified) amusement I may have provided made it worth wading through such a long post.