I would just upgrade the lights and grow a lot of different plants. Botanical filtration is the most sadly overlooked factor in aquarium keeping. Botanical organisms are the reason why the planet itself is as hospitable to animal life as it is. Plants remove countless bad things from the water, and add a more natural setting that will make your fish a lot happier.
Heavily planted systems don't even need water changes as far as the fish are concerned. I go weeks without them, and when I do make the water changes, it's more for the benefit of the plants than anything else.