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I would be worried about your crab attacking a bottom feeder fish.The crab could eat all the un-eaten food anyways to yeah.The spots in the tank might be some type of algae,my guess is slime algae.
80 sounds like a great deal to me for a 60 gal with hood and light, i live in seattle and all the tanks at the different stores that i have been to cost at least a $100 for just the tank, and glass in definitly the better choice