i would go with a topfin filter their cartridges are one of the last of their kind, the to-be-assembled ones, you can skip the carbon and just use the cartridge with no carbon it it, or just stuff it with polyfil floss, plus theyre mostly under $25 for a 10g or 20g rated.
Don't really sell topfin in my area an none on amazon either. Most of the used 5g I've seen around my area come with a Marina i25 internal filter. Basically an all in one media cartridge. That's why I'm probably just gonna stuff it with seeded filter floss. If it comes to me having to buy one I was looking at this one or the aqua clear mini.
http://www.amazon.ca/gp/aw/d/B000SP...qid=1381272439&sr=8-14&pi=AC_SX110_SY165_QL70
I'm more concerned about heaters though. I'm having trouble finding a small but accurate heater for a 5G. I've been looking at elite heaters since they're cheap but they don't have a temperature dial, just one that I have to play around with until I get my desired temp which could take days.