Help: New Fish Don't Get Along?
Hi. I have a 45 gallon tank that has been set up for about three months. I bought it already set up though, so I don't know how long it has actually been set up. I only took the water level down for a couple hours to move the tank. It came with a plecostomus and several different tetras. I proceeded to add fish. I got two koi swordtails, a flame dwarf gourami, an opaline gourami, and a blue gourami. A while later I added a pictus catfish. Then I added an angelfish and a green severum. The last fish I added is a black ghost knife.
The problem started when I added the angelfish and severum. One by one my gouramis got to where they wouldn't come out to eat, and then they just sat there, and then died. Just this morning, I found that my mail swordtail had died. He had been acting sluggish for a couple days. I thought there must be something wrong with my tank, so I had the water tested, and the pet store said all my levels were fine.
Now I'm thinking maybe my angelfish or severum are picking on my fish to death. I don't know what to do. The tank was fine until I added those two fish. Do I just have the wrong tank mates? Is it possible it is a disease in the tank? I'm hesitant to treat for disease because I have read that the ghost knife is very sensitive to medications.