The Problem
A properly cycled tank should have undetectable levels of ammonia. Even at low concentrations ammonia is extremely toxic to fish, and can quickly cause stress and physical symptoms such as inflamed gills, eyes, or fins, lethargy, decreased appetite, or even death.
Treatment & Prevention
Regularly use a water quality test kit to measure ammonia and nitrite levels. If ammonia levels are higher than 0 ppm, do a water change as soon as possible. Reasons for ammonia spikes in established fish tanks include: 
- Too many fish: The smaller the tank, the less water there is to dilute wastes. Doing water changes more frequently and increasing biofiltration capacity can mitigate the problem, but the real solution is to avoid overstocking your tank.
- Using chlorinated water: If an ammonia spike happens after a water change, it may be due to disinfectant chemicals in your tap water, which can kill off beneficial bacteria in your tank. If using tap water in your aquarium, be sure to treat it first with a water conditioner product that removes chlorine and chloramine.
- Replacing filter media: Aggressive cleaning or discarding old filter media gets rid of a lot of beneficial bacteria, so the remaining bacteria may not be able to break down wastes as efficiently. Be sure to monitor the tank closely and do frequent water changes until things stabilize. Do media replacements sparingly, keep a piece of old media to “seed” new media, and clean media with a gentle rinse in tank water.
- Inadequate cleaning: If excess food, dead plants, or dead fish matter are not promptly removed from your tank, they will decay and release ammonia. Be sure to inspect and clean your tank and mechanical filtration media .
Ammonia spikes can be stressful and frustrating to deal with, but looking out for these common issues can help you to troubleshoot and solve the problem. Using MarinePure® biofilter media, with its huge amount of accessible surface area, allows the hobbyist to have higher fish loads in a tank, and still gives room for additional bacteria growth when an upset occurs.