How many clowns were you looking to add? Clowns generally do not do well in big groups, unless you were thinking of doing a clownfish harem tank which would require multiple anemones to balance the aggression. I am by no means an expert on harem tanks or keeping a clown dominated tank so someone might be able to fill you in more than I can. But I will say typically 1 to 2 clowns are kept in tanks will great success and there are many beautiful fish you can keep with a 45 gallon. Check out dwarf angel fish as they are brightly colored and pretty active fish, as well as wrasses. I have a Leopard Wrasse that I love to watch dart around the tank. Another cool option could be a goby/shrimp pair. They have a symbiotic relationship where the pistol shrimp burrows in the sand to make a nest and look for food, while the goby faces outwards in the burrow alerting the shrimp of danger. It is a very cool relationship and something different from the typical clownfish/anemone symbiosis. Whatever you decide on there are plenty of experienced members on this forum to help answer any questions you might have! Let's us know what your decision ends up being and if you haven't already start a build thread to update us