I posted a problem I had previously with Guppies in my thank. Basically they were turning white and dieing. I received various advice all of which I followed. I still seem to be having a problem. I took a dead one to the aquarium center and some water. They said that the fish looked like it had a bacterial infection and dropsy. My water was tested and no problems found. They advised to leave a week to make sure no more fish died, which I did. Last week I purchased some more fish and added to the tank. I also did a water change at the same time. I purchased 9 females. So I had 12 guppies, now 5 have died. Tested water again no problems. There are no markings on the guppies. Before they die they swim upside down, they start to drop to the bottom then pull themselves up but eventaully they drop to the bottom stay stay there and die. There are no visible signs of disease. My other fish (10 neon tetras, 2 clown loach & pleckie all fine and not dieing). What is wrong? My tank set up is = 96 Litres, I use ROwater, sand substrate, live plants. Plants growing well, water tested thoroughly and fine. The only 2 things that spring to mind is that I have a male guppy (added as first batch) that seems aggressive & it seems that the problems occur when I change the RO water. Can anyone help? Sorry about the long post