You don't need a bigger filter you need more surface area for your biological load. Are you using the filter cartridge that came with the Marineland hexagon? If so this is what I would do. Take the cartridge out and cut the filter floss out. Dump the carbon that is inside in the trash. Buy some aquaclear filter foam. Wrap the filter floss around the foam and place it in the filter. Leave the filter floss wrapped around the foam for a few weeks or until your ammonia levels remain consistent. Once your ammonia levels start to stay zero you can remove the old filter floss.
You might still have a lot of space inside the filter box. If so you can add more sponges or you can try adding biochem stars. [ame="http://www.amazon.com/API-731A-Bio-Chem-Stars-20-Count/dp/B0002568M2/ref=sr_1_1?ie=UTF8&qid=1348535737&sr=8-1&keywords=biochem+stars"]Amazon.com: API Bio-Chem Stars, 20-Count: Pet Supplies[/ame]
I used these in a new filter and saw nitrates in less than 2 weeks. BTW I got a couple of aquariums off craigslist the guy threw in a bunch of stuff, one of them was the filter found in the marnineland 5 gallon. I drilled a few small holes in the back of it and pushed some suction cups into the holes and stuck it on the back of my 29 gallon tall tank. I stuffed it with some foam and biochem stars. I bought the missing biowheel. It seems to be doing a good job filtering a bigger tank.
Last edited by goldfishyman; 09-24-2012 at 08:40 PM.