Hi. I have a 160 litre tropical fish tank. I have mostly platys, all different colours, and babies but not a terribly large amount. i also have a molly who tends to chase female platys around. i also have 5 guppies, who are quite placid, they don't cause any trouble, in a separate fish hatchery box i have 19 baby guppies, they don't produce much waste at all. i also have, three cat fish, an eel,three flying foxes, two black widows, a fairly clear fish with an orange/red tail (not sure of the name), another partly clear fish with a black and white striped tail and a rosy barb. The Ph in my tank is fine, there is plenty of oxygen in the tank also, in a few weeks three platys have died, one of which had a slightly brown tail and some white spots on it's side, thinking it was fin rot, i treated the tank with multi cure. a week or two later more died, some are missing parts of their fins and tails which i presume is from the rosy barb who is known as a fin nipper, either that or it's the white molly who is always giving the other platys a hard time. I haven't put a large amount of fish in all at once so it can't be an unexpected waste amount for the existing fish. i have a reasonable amount of live plants to supply extra oxygen. a lot of the older, larger platys will float at the surface or in a quiet place and just sit in one spot and not move, they will breathe and move their fins, and that's all. Could someone please tell me what's wrong with my tank. Also, my rosy barb, tends to do "somersaults" in the water at like 50km/h, it's very strange, sometimes it will swim or float upside down, but there doesn't appear to be anything wrong with it, and most of the time, it swims quite normally, it's been doing it for months.