Depending on the crab, but if they get hungry enough, they are known to eat anything. A lobster will eat fish, so if you are looking at getting one, it would be better to get a tank specifically for them. Damsel's can turn into wholly terrors in a hurry, so unless you are getting very aggressive fish to go in with it, you might want to pass on them. Powder blue tangs like to swim, a lot, as most tangs do. A 75 gallon is going to be pretty hard on them. The tangs in the family of the Zebrasoma(yellow tang is an example) or Ctenochaetus would be a much better choice for a 75 gallon system.
As for corals, some easy corals that won't require a lot of light are mushrooms, leathers, paly's, xenia, green star polyps, toadstools, or kenya trees to name a few.
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