as for crabs the only ones i might recomend would be small hermits, if they dont get new shells theyll kill snails for them, yes you can have to many, they dont pollute like fish bit there is only so much food that they can eat. i have heard one crab per gallon, so a 26 can have 26 crabs, if anyone knows another rule let him know im not sure on this. Other larger crabs will catch and eat small fish, so be careful and reasearch