Please help me my fish are dying! I have had them for about 8 months and I bought a new and bigger tank. It is now about 120 litres but the fish are now dying!! In 4 days, 5 fish have died! The temp. Is at 73F and the internal cartridge filter is working well. I have dechloranised the water and tested the waters levels. I don't see what I am doing wrong? Please help!
Sorry for the late reply. I have been busy in irl.
The cause for fish dying can be a number of things / possibilities.
1. Did you change the filter when you upgraded the fish tank? If so, the new filter media has not been cycled, which could cause ammonia spike. If it was the old filter, if you let the filter media stayed out of water for too long it can also cause the collapse of the beneficial bacteria colonies. The result would be the same as getting a new uncycled filter.
2. When you transferred your fish to the new tank, did you also transfer the old tank water? If the fish were suddenly put into all new water, they can get into shock due to the sudden change in the water perimeters. Fish can get into shock and even die when the temperature, PH, hardness, and even nitrate concentration is too different between the new and old water. It would be a good idea to use the old water in the new tank from the beginning. You may add some new water, but let the fish have enough time to acclimate / adapt during transferring.
3. Did you add new fish to your new fish tank? If you have added too many new fish at once on the top of the existing ones, it may cause ammonia spike.
4. The water temperature is a little on the low side. I'd up it to 76~80F.
Tropical fish are dying! 
