Why isn't my betta eating?

Ok I know that I fed my betta 1 pellet of his food. Usually he gobbles it up before I can. Blink. Yesterday he didn't touch it and let it go to the bottom. His tank is rather dirty so I cleaned it right out. I made sure the water was de-chlorinated before I put him back in. I tried feeding him last night after I cleaned his bowl but he still wouldn't eat. So I took out the pellet and threw it out. This morning he hasn't eaten for a day now so I give him a pellet. He doesn't eat it so I take it out. So a few hours later I try something different. I crush up a pellet and give it to him. He goes up and just swims near it. Still doesn't eat. He has never done this before. What could he wrong? How can I fix it?