I have had a disaster with my tank and hoping someone can help or offer advice for the future. I have a 90 litre tank. I did a water change at the weekend. I use RO water. I also added 4 Guppies. Before I added the water I noticed it was very cold, so I rang the Aquatics center who advised me to wait a few hours for it to warm up which I did. I added the water but noticed that when got to the bottom of the barrell that there was what looked like lupms of chalk (turned out to be the minerals). The water and fish were added Sat afternoon. Last night I noticed that the fish were all swimming to the top and shoaling where the filter(external) pumps the water back into the tank. A couple of fish were struggling at the bottom of the tank. When I woke up this morning the fish were dropping like flies. My 2 clown loaches were covered in white stuff, which the aquatics centre said was mucus. I did a water test I will give the reading this from this A.M. and reading on Friday in brackets. PH was 6 (7.6), High PH was 7.6 (8.4), Ammonia 1 (0) Nitrates .25 (0) and Nitrites 80(40). I changed the water as it was 4 weeks from the last change. Even now as I am typing I have lost another fish. The only other thing I did was to wash the filters from the pump in a bucket (clean) of the tank water to get rid of excess rubbish. Tank temp is fine. Any help would be greatley appreciated.