My astraea snails are thriving. More algae than 2 can handle and there were 2 spare shells for itMaybe your just feeding the inhabitants with new crab ? Just a thought.
do you have shells/ spare shells for your crabs to move into as they grow,?
So many questions and answers I assume will be mainly your aquarium is quite new, you may think your providing food, crabs do love abit of algae which I assume your aquarium has none