High ammonia levels in an aquarium can be the death knell for the majority of your fish, and it's therefore essential that you keep the levels either low or non-existent. Ammonia levels must be lowered whenever it reaches 1ppm (parts per million) or more, as can be determined by a commercial ammonia test kit available at most pet stores. Luckily, there are several ways to reduce the ammonia in your tank; however, you may need to perform all of the methods to appropriately lower its level.
Remove any dead fish or dying plants from the aquarium. The most common cause for an ammonia spike in aquariums is the death of a fish. Once it dies, it will begin to decay and release ammonia. Regularly check for dead fish and remove it immediately with a net. Also make sure to remove any dying plants, as these will also add ammonia to the tank, though admittedly not as much as dead fish.
Reduce the amount you feed your fish. Uneaten food that is allowed to dissolve or spoil contributes to the ammonia levels in your tank, as well as the food that gets caught in the gravel. Only feed your fish what they'll eat, as overfeeding will always lead to not only an ammonia spike but cloudy water.
Use an ammonia remover, which is typically composed of liquid drops that you add to your tank according to how much water you have (the correct amounts should be labelled clearly on the packaging). Note, however, that ammonia removers are not long-term solutions and should be used only while you work to alleviate the ammonia problem permanently.
Perform a 30% water change. Make sure to replace the old water with fresh, de-chlorinated water that is around the same temperature of the water already in your tank as not to cause your fish any shocks.
Check your ammonia levels again after 24 hours. If they have not gone down, perform another water change, except this time only change about 20%. Repeat this process as needed until the ammonia has reached the desired level.
Never change more than half the water in your tank at once. While this may seem like a convenient solution to an ammonia problem, this will shock your fish, causing stress and potentially death.