Any smaller saltwater fish that are easy to care for?

[KellyFrancis]

I agree with the go larger advice, even if its only 10 or 20 more gallons. You have to keep in mind the water you are displacing with the rock, so you'll really only have like 7 or 8 gals of water to play with. To give you an idea of the evaporation, over the summer, I had a gallon a day evaporating out of my 55. And I've got two glass covers on it. Not a whole lot was exposed to the air. lol. But you could do a sweet little coral/invert setup with a small tank like that. Personally, I love my inverts. The CBS and Nass snails being some of my favorites. Really fun to watch, esp at feeding times.

 

[Wiggles92]

Use two ten gallons since it sounds like you have multiple spare ten gallon aquariums. One will be the display aquarium, and the one will be the sump. You will save yourself a lot of hassle and will be able to maintain a more stable system.