A 55 gallon would enable you to have a bimac (O. bimaculoides), which a number of our Tonmo members keep. This species is captive bred in California and you can get quite a young one. Have a look at some of the photos under Journals and Photos here at Ceph Care.
If you're interested, then we have lots for you to read - just click on the Ceph Care button above and look at our articles, starting with the Equipment List and the Checklist.
If you have further questions, please don't hesitate to ask.