I've been keeping and breeding a wide variety tropical fish for almost 30 years and I still have one of my first fish! One of my aquariums is an Aqua One with its generously-sized external filter as supplied. It's been running for 4 years now with regular maintenance and an established piece of bog wood complete with Java Moss, some rocks and a fine layer of river sand sprinkled with coral sand to act as a pH buffer. About a month ago, I added a tall plastic aquarium plant with a gravel/resin base, and on 12 Sept, did my usual 50% fully treated, slow water change. As my tank is slightly understocked but with small traces of algae, on the 19/9/10, I bought 3 small Bristlenose Catfish and added them to the tank with one dose of Myxazin as a general precaution. They immediately started removing the Algae traces, so a few days later I added a few small cucumber slices. About 1 week later, I noticed most fish were spending most of their time in the top water layer but they did not look distressed and were feeding. On switching the tanks lights on the next morning, however, all the fish apart from the Bristlenose Catfish were dead (6 adult Australian Rainbow Fish and 2 Clown loaches). All 3 Bristlenose Catfish looked ok then but despite removing the dead fish, the catfish also died a few hours later. All the fish were stiff but had no obvious symptoms and were in full colour, though the catfish had slighter more mucous than you would expect from recently dead fish. On checking my water, it appeared very clear and normal with no surface films, temp 75 deg F, pH 6.8, zero Nitrite and 10 mg/L Nitrate. I've beem monitoring my heater for the past few days and it's controlling as normal. If I had a Nitrite peak it must have disappeared very quickly but the catfish could have been carrying a nasty disease, the cucumber could contain pesticides on the skin, and I can't rule out the plastic plant which might have toxic epoxy residues in the base or even my public water supply. Has anyone lost all their mature fish in a mature tank as suddenly as this? It could have been Nitrite or a faulty heater but I'm thinking of stripping the tank down and maturing it from scratch to eliminate any nasty diseases or toxins.