Sounds like u have three possible issues
1st. You have an algae bloom. This is caused by over feeding and/or too much light. Cut back on the feeding, vacuum the tank and check the levels again.
2nd. The ammonia shouldn't be there at all. If your not using liquid tests, it's common to get a false read with ammonia. Before you drop a chunk of cash on a filter get a liquid
test kit, that is if you don't already have one. If you do, your cycle is messed up and you need to read up on fixing that.
3rd. As stated above your tank might not be cycled or it could have started over. How old is the set up and what method of nitrogen cycling did u use?