1 cubic foot = 7.5 gallons
2.5x2x4 = 20 CF
20 x 7.5 = 150 gallons
You could do very well for tangs, chromis, a lion fish, some species of puffers, triggers, etc in a 150 gallon tank (not all together, just giving some suggestions). Just not sharks or rays. It sounds like you're new to saltwater fish keeping?