Filter troubles please help?

i have a tetra 20 gallon regular starting kit. it has been running for about a year now. usually when i do a water change, the filter cleans the water well enough and it is clear within a day or even a few hours. I recently did a water change, (2 days ago), and the filter seems to be performing less and less effectively. i cannot even see the back of the tank and it is barely 12 inches deep. what do u recomend i do? i saw on foster and smith aquatics that there is a hang on back canister style filter. i would like to get this as it polishes the water apparently pretty well, but is it good? i want a canister filter but would rather not go through the troubles of setting one up and the cost of the filter. i could do a canister but if you have a great filter that works (and fits) with this tank please tell me.