for a 55gal if you wanted to keep some of the more sensitive fish and what have you, you might want to look into purchasing or
DIY sump filtration system. A sump would replace the filter you currently have and is a great place to put other chemical or mechanical add ons (like a protein
skimmer). The system itself is relatively easy to understand and install and greatly improves the quality of your tank water. If you include a specialized compartment in the sump called a
refugium you can grow your own critters (amphipods and copepods) in greater numbers which act as a good source of fish food and are used as the primary source of certain fish.