| | Diseased Fish
Folks: My biggest problem in keeping aquariums has come from infected fish that I purchased in stores (or on-line). My experience has been especially dismal with store-bought fish (nematodes, ich, Fish TB, etc).
If you think about it, you can see why. Thousands of fish come into a store from all over. Even if only a few are carrying disease, the disease can gradually spread to healthy fish.
Many imported fish carry fish TB-- some TB strains are much more virulent than others. My Rainbowfish contracted a nasty one in 2004 (brought in by new fish purchased on-line) that I've managed to bring under control with UV sterilizers. Fish have stopped dying from it.
I recently corresponded with a vet who works with Asian guppy farms [these farms have (and have had) big problems with fish TB]. Vet would not comment when I asked him what these farms do to make sure that the fish they're selling are healthy. From his "no comment", my impression was that these farms just keep sending their infected fish. After all, infected fish can look just fine for a couple months before they succumb to the TB or secondary infections (TB will compromise the fish's immune systems). The miracle, in my opinion, is that one ever gets a healthy fish from a store. It's a testimony to the fish's immune system and just dumb luck.
There was one well-managed aquarium store where I was (in 1987) able to get healthy Rainbowfish, cichlids, etc. The owners were dedicated hobbyists and knew what they were doing. Even then, though, the imported guppies they sold were infected at the production source. I know this because I tried purchasing guppies that had never passed through the store's tanks. [The store would call me when their fish came in, and I would immediately drive over to purchase guppies still in their shipping bags.] Despite the best care, these guppies all died within a few weeks. I assume (now) that they were carrying fish TB. [I wish that this store had stopped buying guppies from a vendor that was selling diseased fish.]
I doubt that things have changed much since 1987; there's just no accountability. I quizzed (recently) one on-line, retail vendor that advertises that it sells healthy fish. Many of their fish were advertised as coming from Asia. I asked: How does it (the vendor) screen its fish so that the fish I was purchasing does not carry Fish TB?
The vendor's answer was that they quarantine the fish they import for two weeks. That is the extent of their "screening". Fish TB takes a few months to manifest itself, so a two week screening is false security, indeed.
Bottom Line: Fish deaths are not always due to poor care. I would advise beginning hobbyists to (if at all possible) buy directly from the fish breeder.