couple questions

I have a couple questions I'd like to run by people for advice. Does anybody have suggestions about the best way to clean a used tank? It looks raunchy and I don't want to leave any harsh chemicals behind.

2nd question: How much flow is too much? I'm considering getting a second power head for my 40 gallon breeder to get to some of the dead spots, but don't want to overwhelm my fish.

Thanks and merry Christmas.