I have a 125g....with a rena xp4 and aquaclear 110....Cheapest would probably be a diy wet dry filter, most expensive part will be the pump. If you search the forums you will find plenty of diy plans. Or you could get away with a few hang on the back filters such as an aquaclear 110, or a emperor 400. I also suggest buying the filters etc.. online it will be much cheaper than in a petstore unless ofcourse you get a five finger discount.:laugh: Good luck, also from experience it's better to spend a little more on the filters etc, because usually with the more expensive ones the build quality and reliability is much better, which will save you money in the long run. I highly recommend anything from eheim you won't be sorry.