sells the following 30
gallon "marine algae pack:"
6 scarlet hermit crabs, 15 red leg hermit crabs, 20 margarita snails and 5 turbo snails
For 15 gallons you could use half of this, but this will depend on factors such as amount of rockwork, amount of algae (the food source,) etc. Personally, I have found a smaller clean up crew to be adequate for my 30 gallon aquarium.