I have a divided 10 gallon. I do a 50% water change once a week and I vacuum both sides fairly well. Each week there's so much debris that it looks like I haven't cleaned the tank in a month! The side without the filter is the worst. Both males eat every pellet I give them, so what's causing it? At the end of the week, I can see it collecting on the plant leaves (fake) and other flat surfaces. When I just had the one male, I only noticed more coming up the gravel vac. in the area of the tank that he stayed in the most. The tank light is 2 15w bulbs, but honestly, they're hardly ever on. I either forget to turn them on, or they're on for 4 hours or less. Am I doing something wrong?
It's a HOB filter that came with the 10 gallon tank. I'm not sure exactly what kind it is, but it definitely can't move. I don't mind cleaning all of that out of the gravel and off of the plants, as long as it's not too harmful for the fish.
Is your filter fixed in the center of the back of your tank? If so, you might want to consider a double divider with the heater in the middle bay, too. It'll even things out and, if you can increase the filter flow, may improve the filtration.
I have found that pretty much the best way to filter a divided tank is to put a filter in each side. I would suggest using Tetra Whisper 3i internal filters because you can run two of them off a single airpump using a gang valve. You can also use little box filters like Lee's or Penn Plax. But if you don't filter both sides, one side gets stagnant and nasty.
Please don't spend your money on Tetra 3i sponge filters. They are not properly designed to filter anything. You can see, by running one with the foam out, that the air/water lift pipe completely misses the filter chamber. I can't believe they persist in selling this bogus device.
They can be modified to make it work, but why should anyone have to do this?
If you're going to go with a new sponge filter, get a good Hydro '0' or equivalent and put it between two dividers.
Having said that, I don't like sponge filters...too noisy.