I just bought a tank at a garage sale for $6. How can I tell what size it is. I thought it was a ten but I'm not to sur is ther such things as 15??? And how many crabs can I keep in there???now I only have 2 and they are at my daycare, but I am planning to bring them home for more and better care thanks to this website. I am officially a crabber. I love these little guys!!!