Buying a marine aquarium
...most fish stores cannot stock the size of tank appropriate for most of the fish they sell. Tanks of 30 gallons or under are appropriate for a clown pair, a few gobies or blennies, a royal gramma or dartfish, and some inverts. And with good lighting---corals.

If you want larger fish, you have several size jumps. A 50 will let you have a dwarf angel or similar, anything a 30 can handle, maybe a single chromis (a damsel, like a clown, mildmannered,...