An ammonia and nitrite spike in a well cycled aquarium can only mean one of the two things:
A. Bacteria colonies in your filter media have collapsed or at least being damaged.
This can happen when you have used some strong medications for your fish. Or perhaps you have washed the filter media too “clean” by using untreated tap water or squeezed it too hard and too many times. When you have fewer bacteria doing their job of converting ammonia and nitrite to nitrate, it is easy to understand why there is a surplus of ammonia and nitrite in the fish tank.
Jan 21, 2013 - How to remove nitrate from the aquarium
You will first need to determine what the nitrate level is in your aquarium by testing the tank water with a test kit. Follow the directions in the kit, and by matching the color of the test with the included color chart, you will know how much nitrate is in the water. Although many aquarists run their tanks with extreme nitrate levels, the ideal is a maximum of 5 to 10 ppm. Levels of 20 to 50 ppm are too high. Freshwater tanks can be at the higher end, with marine fish-only setups at the lower end and reef tanks as near zero as possible.
High nitrate can cause severe algae outbreaks and green water
Purchase biopellets from your trusted fish supply source and add them to the tank as instructed by the manufacturer. Biopellets are a relatively new aquarium technology harnessing the natural power of beneficial marine microorganisms. The pellets dissolve in the water, and all the microscopic critters inside feast on nitrate and other undesirable water components.
Are nitrates harmful for my aquarium? | Algone