I recently got some Platies and I didn't quarantine (I know- I should have) and 3 of them died within 2 weeks. So I was worried that they had a disease from the store but all of my other fish were ok. But today I found 3 of my serpae tetra dead and 2 of my zebra danios. Now I'm afraid that they spread a disease. The fish didn't look sick that I noticed. I would just find them dead in the tank. I have 10 serpae tetras, 2 Platies are left, 4 zebra danios, 2 Otos (I'm going to get more. I bought 4 and one was DOA and I lost the other one when I redid my tank), 3 amano shrimp, and 5 leopard cories (I just realized I'm kind of attached to those) so now I'm scared all of my fish are going to start dying. Is there a medicine I can use to dose the tank that will kill whatever disease is in there?
My tank is 55 gallons, and the water parameters are normal.
ETA: I'm pretty sure one of my serpae tetras is in the process of dying right now. Its mouth is wide open and it looks like its stomach is slightly swollen.