Help! Freshwater Aquarium Questions?

Ok, I've kept a freshwater aquarium in the past and withing the past 3 months I set up a new one. Well, everything was going just fine and about after 2 months, I added a pecostamus fish. I had had one before and loved it. It helped to keep my tank clean. Well, I went in there this morning to find it belly up! I was shocked! I've never had a fish to die that quick! I've had it about 2 weeks. I've always had great luck with fish. So, I tested the water, as I do every week, and the ammonia was sky high! I know it will raise when you have one to die. But for some reason, the aquarium, especially the gravel is filthy! It looks awful and I feel so bad for the fish I have in there. I have 3 tetra's in now. How can I clean the tank and the gravel and get my leval's back to norm? I mean, the gravel is really bad! I've never had this happen. What's up???? Please help me! "_