Ammonia is the first step in the Nitrogen cycle. It's a sign of decaying waste, food, dead fish hiding somewhere etc. Bacteria turn ammonia in to nitrites, and a different bacteria turn nitrites into nitrates. You can add a heavy plant load to remove nitrates, or do water changes to remove/reduce nitrates.
If your filters can't keep up with the ammonia/waste either you're feeding too much, or there is something else causing the ammonia spike in the tank. ie what substrate are you using? some substrates will release ammonia.
In an emergency situation massive water changes can be used to reduce ammonia, but you're taking food away from the bacteria living in your filter media so you don't want to rely upon it to keep your ammonia levels down.
While doing a water change are you vacuuming your substrate? if it's gravel are you inserting the tube into the substrate and pulling as much crud up as you can? (only part of the substrate for each water change, as bacteria live here as well)