We've kept temperate fish in a 57L tank for about a year, slowly adding more fish, shrimp and snails, and tried to upgrade to a 120L tank this week. We carefully stored water for weeks/days (which is reverse osmosis from our drinking tap, and added some 'replenish' - or called something like that). We used all the old tank water, and really slowly moved everything out - plants, ornaments, extra filter medium that we'd placed to habour more bacteria. We'd allowed the old tank to become really dirty (embarrassed!) so tried to rinse some things but not most as we knew to keep the bacteria. We just got our first cat a couple of weeks ago, which hurried the move as the old tank wasn't very safe from her. We had completely new gravel, as didn't have enough old and some extra ornaments (is this the problem?) , which was carefully rinsed many times. We used more new water than originally planned as the pump needs the tank to be full to work (stupid design!). We added some bacteria solution to the filter. So all was fine for about 24 hours, and then suddenly first the only ghost shrimp died, then the first zebra snail, stupidly didn't twig. Then eating dinner hours later and suddenly all the fish were on the surface gasping. Two dead. 6 out of 8 armano shrimp dead, all snails dead. Quickly got some fresh RO water from the tap and fished out two, who immediately revived. Waited few minutes and they were definitely better so got the rest out. Some still suffering a bit, but before getting them out, thought more were dead as they were still. The ones left are: Leopard danios x3 Zebra danios x3 penguin tetra x1 (still suffering) red eye tetra x1 white cloud minnow (seem the least affected) x5 Think the others are glowlight danios x4 (not sure on this one) Did have other red eye and penguin but lost them slowly months ago. Took some test water (straight after we removed the fish) to the local store and they said nitrite and ammonia were fine, only thing was slightly low bacteria and slightly low pH (which they said would be linked??). Left the filter running, and used our testing kit (only three parameters) about 2 hours later and pH was 7, ammonia 0, and maybe slight nitrite. We haven't plugged in the heater, and the light is LED, but it felt warmer in the new tank (will get a thermometer!). Questions: So the fish are currently going back in the old tank (in bucket till it fills up as out water is slow), no plants, all ornaments BUT do I add some of the new water or leave them in entirely RO water? Or do I use our softened tap water? What did we do so wrong? If it's not nitrite or ammonia, what's the problem? Was it the new gravel? New plastic ornaments - all rinsed very carefully. We did add a few new fish and two new plants at the same time. Feel gutted and little daughter is distraught by any death in the tank.