Ok this is what i have done & whats in the tank. Tank has been set up for about 8-9 months 100L ph 7, brought this test kit, Tetra test, NH3 / NH4 Total ammonia kit.(about a month ago). tested water yesterday and that is fine. I changed the water (50%, do this weekly to all my tanks) on the 16-10-03, about 2 fish had died by then, started putting Spotoff into tank, but fish still dying, there is no signs of anything all are eating well etc, Haven't got filter in this tank, fish are 6 months old and 3 months old. Plants are Java moss and have been in there from day 1. Nothing new has come into the tank. When i find the fish in the morning dead they are covered in white fungus all over body and are floating on top. I have a total of 18 tanks and all the rest are fine. The tiger barb is in a different tank. On the bright side still have 2 Plattys left none have died for 2 days