I would go for a HOB
(Hang On Back) filter. I'm not a big fan of internals myself. They take up precious room, especially in smaller tanks. They can be a pain to clean also. There are some very good HOBs out there. Do a bit of research and pick a few, then get peoples opinions on them. I can't help you with brands. We have different brands over here. Whisper, aquaclear, fluval are all very popular and very good filters. Stay away from cheap HOBs from china(ebay), they are crap, noisy rubbish. You get what you pay for!!
You'll be surprised how bigger the tank looks without it....lol