I believe filtration is misunderstood by many aquarists, other than the population of bacteria on bio media using ammonia and nitrite, and the filter grabbing particulate for the aesthetic viewing appreciation of the aquarist, commercial filters are basically smoke and mirrors.
Water changes are the most important factor in any aquariums, and its fishes health.
With your species and stocking level, you could easily be removing over 50% of the tanks water and replacing it twice per week.
Just replacing evaporation and a little more, is the process of concentrating urine and other metabolism by-products creating a thicker and thicker fish urine soup.
And normal aquarium filters, do nothing to remove that " fish piss soup".