Prime doesn't eliminate ammonia. It neutralizes it temporarily - for about 24 hours. After that it reverts back to its toxic state and will kill your fish. It's awesome that you have nitrate in your tank already though. That means your tank is almost done cycling (I say almost because you still have ammonia). Definitely cut back on feeding for a while, the fish will be fine without food (or with less food) briefly. Maybe try feeding them only what they will COMPLETELY consume in around 1 minute - and feed them like this only every other day for the next week or two.
Keep doing those water changes. It is, by far, the most effective way to lower that huge amount of ammonia you have in your tank. Once you get ammonia levels under control you can start to cut back on water changes.