The beneficial bacteria do not reside in the water column. Their colonies are established in substrate and filter media, not the water column thus you are only trying to move the usual dirty water than actually helping with the nitrogen cycle.
Hmmm..Please let me know if it's still difficult to understand.lol My own perspective of word comprehension differs.
I mean aside from the filter you have right now, add more established filter media to ensure, whether your test kit is inaccurate or not, that the nitrogen cycle keeps the ammonia and nitrite at bay. Both are toxic to fish in high levels. If your pH is low, then ammonia is not an issue as it turns into ammonium. It will reactivate its toxicity if the pH is increased above 7 but pH is hardly an issue. Focus on making sure your ammonia and nitrite are zero.
When I say pathogens, it entails flukes, parasites, protozoans and Aeromonas bacteria that are responsible for the various illnesses of the fish such as columnaris, finrot, etc. Prevention is the best cure after all.
