|Bio-film of algae and bacteria is forming on the bottom of the pond|
With all this positive water filtering, I adhere to one of the traditional ranchu keeping rules - weekly 100% water changes, or in my case more likely 85-90 %, as some water remains in the sump tank. With working filter ammonia and nitrite levels are very low, so my water changes mainly reduce nitrate level, which can't be utilized naturally and effectively in my existing system. I use municipal water and do not add anything expect the water conditioner to neutralize mainly chlorine and chloramine, which are deadly to the fish.
Even though the water can be tested for ammonia, nitrite, nitrate, pH etc. and show the "norm", there are other signs that can indicate water quality shift. Smell, color and foaming on the surface, along with how the fish behaves are all great indicators if the water change is needed. Daily fish observations are so important along with knowing how to read their behavior. Still, so much to learn here!