I used white vinegar to clean my tank. I don't know how much. I basically just dumped some vinegar in, used a wash rag and scrubbed the tank, the plants (plastic), the castle, heater, filter and the hood. Then I rinsed. Then I got them with vinegar again. Then I rinsed a couple more times. I put all the stuff in front of a southern facing window for a day and a half to dry.
I have my tank set up again. First I put the snail in. He seemed fine. I waited a good 5/6 days before I put another betta in. He seems to be doing just fine as well. I'm not sure what King Fishy died of. I suspect he was sick when I got him. If you look at my posts, I had trouble with him from the beginning. I spent about 45 minutes picking out my new betta. He was lively, brightly colored, no tears/rips/black spots on any of his fins, no white specks, no white cottony blotches, no gold dust on the gills and he had a ton of bubbles in his cup. He's a fiesty lil devil and swims non stop. I hope this helps someone else in their search/cleanup/care of a betta. I hated to lose King Fishy, but hopefully his passing can educate others.