What are nitrifyers?

Why would all your fish die in the last two days?

There may be bacterial growth not compatible with the type of fish you have. Make sure your tank stays relatively clean. Try using an algae eater to clean up the mess. Make sure you feed your fish. Do not overfeed them, and do not underfeed them. Make sure there is adequate oxygen supply with an air pump. Don't use house cleaners or other harmful substances in or around the fish tank. Make sure you use the right kind of water compatible for your fish. If it is a saltwater fish, use saltwater. If it is regular water fish, make sure the water you are using isn't full of iron or other metals commonly found in well water as well as chlorine and other chemicals found in tap water. Try filtered or bottled water, or use a special balancer you can find at any fish/pet store that will create the best habitat for your kind of fish. Make sure each of the fish are compatible with one another. Some fish will kill or eat other breeds of fish as part of their nature.