We have a 20 gallon tank. It has 4 platys, 1 cory catfish, 1 otto catfish, 2 tetra fish (don't know the specific name), and 1 pleco fish. The temperature in the tank is about 69F, and the water (tested by the pet shop) was said to be perfect. The tank is established, with beneficial bacteria.
Well, one week ago, we had one neon tetra fish. We also had an algae problem. What we did was went to the pet shop and bought two neon tetra fish (to form a small group) and a second otto catfish to help the algae problem. First (about a day later) the new otto catfish died. Two days later, the two new neon tetras died. We think at least one of the three new (now dead) fish have infected our tank with some sort of parasite, bacterial disease, etc.
How do we know this? Well, first, the original neon tetra that had been in our tank for a few months now had died... a few days after we got the new fish. The day before that, we added a pleco fish because the new otto catfish died, and we need our algae controlled. I am seeing other symptoms of sick fish. Yesterday, my platy (I believe only one platy is doing this) which we've had for about a year is pooping clear white poop. I called the pet store, and they told us that it was a parasite. We went to the pet store and bought "Quick Cure" by aquarium products. The package said it is "fast relief for ick and protozoan parasites". The employee stated that it would cure internal parasites, so we bought it. I came home and added it to the tank, following the directions. It told me to remove the filter, so I removed it.
Today, the pleco has been acting funny. At first, I thought it was dead, sitting behind the heater on the wall. I moved the heater to see exactly what had happened. He is still alive, but now he is exhibiting the same behavior as the sick neon tetra fish--swimming upside-down, sideways, etc. I know it is not swim bladder disorder, because we do not feed our fish a lot, and only every two days. The other fish are swimming fine, but they seem like they are a little "shy" today, hiding in places they normally wouldn't hide at.
I do not have any idea what is wrong with my tank. I want to know what disease they have, how to cure it, and what to cure it with. I do not wish for them to die...