lp85253 Is absolutely right, you are going through a bacterial bloom. First hint towards a bacterial bloom was that you exchanged your media. The main causes of bacterial blooms are removing/ destroying beneficial bacteria. I went through a bacterial bloom once because i vacuumed too much of my tanks substrate and set things out of balance. You will need to keep an eye on your ammonia levels, during Bacterial blooms most of the beneficial bacteria dies off, so therefore ammonia wont be broken down.
1. Dont feed your fish for a few days, make sure you get some type of Bacteria additive like stability or cycle complete it will help catch up. The most usefull thing you can do now is to either go to a local fish place or ask someone you know if you can get some crud (the brown goop when you squeeze sponges/ filter media) It is packed full of beneficial bacteria and will help bb establish in your filters by placing it in your filter. Also
2. If you have a way to add more oxygen to the water either by lowering the water level so the water falls into the aquarium adding oxygen, or by adding a bubbler either will help alot.