I think time to start with the obvious:
How old is the setup...if new, take a look at the topic "nitrogen cycle".
1) what is your ammonia level(test kits available just about everywhere fish are sold)
2) If it is nonzero and you still have fish in the tank, then don't get new fish. Use a little amquel to neutralize ammonia and wait out the nitrogen cycle(not long, maybe a week in a 10 gallon)
3) If all your fish died, consider doing the fishless cycle. google it.
How old is the setup...if new, take a look at the topic "nitrogen cycle".
1) what is your ammonia level(test kits available just about everywhere fish are sold)
2) If it is nonzero and you still have fish in the tank, then don't get new fish. Use a little amquel to neutralize ammonia and wait out the nitrogen cycle(not long, maybe a week in a 10 gallon)
3) If all your fish died, consider doing the fishless cycle. google it.