More filtration is not the answer to everything. However since you posted this question in the Equipment section it probably is. Had you posted this in low tech, you would get a completely different answer.
In order to solve the issues your having to best fit your needs, you must first decide and describe to us what goals you have in mind for this tank. You can get away with a sponge filter in this tank. Perhaps you just need to add a power head to drive it. Perhaps you need to add plants or use a different substrate.
Let's see some pics of the tank. Tell us a bit about what your plans for the tank are.