i think discus would look AWESOME in that tank.
Its a freshwater cichlid that does require a bit of special care, and can be a bit pricey, especially if you want to buy them big and not raise them as juvys.
They like lower-to neutral PH, and generally a bit softer water, but can be kept in harder tap water as well, as long as the ph remains around neutral.
They get about 6-7" full grown, and are round fish with a pit of a pointed head, individuals shape does vary with roundness. They eat like pigs (carnivore) and have TONS of personality. The are extremely peaceful and are generally kept by themselves, or with other peaceful, smaller fish such as cardinal tetras. They like higher temperature water, generally around 85 degrees F, although they can be kept in temperatures from 82-88. And will survive, but just not thrive with lower or higher temperatures. Discus can actually withstand temperatures into the low 90's. They are very colorful, and have a wide variety of color from strain to strain, so you said you would like a colorful active personable fish, here it is. 8).
Just thought it would be a cool fish for you to keep in a bigger tank, and it fits a general description of what you want.
Smaller juvys usually run about 25-35 bucks a piece at 2.5"-3.5". Bigger ones are much hardier, but generally run 80 bucks or more, depending on actual size and strand. (USD by the way).
Juvys are fine, but should generally be kept in a speices only "grow-out" tank, because they can very easily be stunted or killed. Generally, when growing out juvinilles a partial water chance (15-25%) is done daily or bi-daily until the fish have reached adult hood at which time it steps down to about once or twice a week (although more never hurts). They are messy eaters, so water changes are a must... and they eat strictly meat which becomes toxic ammonia fast if left in tank for too long.
Some specimens will eat flake and freeze dried preperations, while others wont touch them, and stick to only frozen meaty foods such as mysis shrimp, krill, beefheart (jack wattley has a commercially made discus beefheart frozen cube food that is very good.) brine, turkey heart, as well as others.
Some people make there own frozen discus food, by mixing together beefheart,raw shrimp,white fish,raw liver (typically beef), vitamins such as C,E, and B12, they also may add in things such as carrots, lettuce, spinach broccoli heads, and peas.
Sorry for the long post, i know i left all sorts of information out... but before i continue, i should probably make sure you are interested in this fish 8)... dont wanna bore you with useless facts about a fish you dont wish to keep.
http://www.gcca.net/fom/fom_pix/discus.jpg
i attached a picture of a few discus, so you can get a general idea of their beauty.
-Jay