The effect of medicines and fertilizers on biofilter operation.
Let’s imagine a usual, routine situation: you have a wonderful aquarium, beautiful fish, equipment you didn’t skimp on, good lamps, an excellent external filter… Idyll. And everything would be fine, but suddenly ichthyophthiriasis is brought in with a new fish. You take out Costapur, treat all the fish, and when everyone seems healthy and no spots are visible on anyone, suddenly in the morning you discover a dozen dead fish in the aquarium. What, how, why? After all, you did everything right, so how could it happen that fish that were almost cured began to die? And a treacherous thought creeps in — the medicine must be bad, it poisoned all my fish! But what is the reality?
Everyone knows what biofiltration and nitrifying bacteria are nowadays. So, you have a well-established aquarium, running long enough for 
a biological balance to be fully established, ammonia and nitrite levels are close to zero, and the nitrate level is also low. And in this aquarium, an outbreak occurs, for example, of ichthyophthiriasis introduced with a new fish. We begin to use medicines, but how do they affect other microscopic inhabitants of the aquarium, such as those same nitrobacteria? Since all medicines are essentially poisons, they undoubtedly have an adverse effect on both the harmful and beneficial microflora of the aquarium, if not killing it completely, then severely inhibiting its development. And this very thing can be the cause of the deterioration of the fish’s well-being even after the completion of the treatment course, when, it would seem, the danger has already passed.
In fact, almost all medical preparations suppress the nitrification process in one way or another; those that suppress it slightly are not as dangerous, but some act very strongly, up to a complete halt of the nitrification process. Most of these drugs have a depressing effect on bacterial colonies that carry out the biofiltration process, which means that during the treatment of fish you worsen or completely stop biofiltration in the aquarium, and the filter operates during this period in mechanical water purification mode. Unfortunately, various sources may cite different degrees of effect for the same drug; this is probably due to different water chemical parameters, in particular pH. During the treatment of fish, it is also necessary to remember that many oxidizing agents (KMnO4, methylene blue), almost all antibiotics and sulfonamides are lethal to nitrogen cycle bacteria.
The second point associated with the treatment of aquarium fish is that after treating any disease, the filter and filter media should be thoroughly 
washed, and here you don’t need to spare the bacteria. The fact is that residues of therapeutic drugs, as well as pathogens from which you were just trying to get rid, often remain in the filter or its filter media. Drug residues interfere with the restoration of normal biofiltration, while pathogens, after the termination of treatment, can provoke a re-outbreak of the recent disease in fish.
Thus, after using medicinal products, it is mandatory to apply bacterial cultures to maintain the normal functioning of the biofiltration process in the aquarium. Generally speaking, it doesn’t matter whether the drug had a strong or weak effect; it is still better to pour a preparation with bacteria into the water at the end of treatment, and during prolonged treatment, use it regularly and consciously to maintain the functioning of the biosystem.
There is another component of potential impact on the efficiency of nitrification in the aquarium: fertilizers and micronutrients. Different chemical elements and substances affect the aquarium microflora in various ways, and far from always negatively, which is also desirable to take into account. Below are the most commonly used minerals and medicines and their effects on the nitrification process in the aquarium.
Calcium. High calcium content is important for the metabolism of Nitrosomonas; with a lack of calcium, nitrification efficiency decreases.
Magnesium. Magnesium content is important for bacteria of the Nitrobacter group. However, too high doses (over 75 mg/l) reduce both ammonium and nitrite oxidation.
Sulfate. Sulfates block the nitrification of both groups of bacteria: Nitrosomonas and Nitrobacter.
Iron. Iron at a concentration of 10 mg/l can slow down the conversion of nitrite to nitrate, while supporting the oxidation of ammonium to nitrite.
Potassium. Potassium has a positive effect on the process of oxidizing ammonium to nitrites.
Phosphate. The addition of phosphate ions has a positive effect on the oxidation of ammonium to nitrites.
Manganese. Manganese positively affects the oxidation process of both ammonium and nitrites.
Chloride. Chloride has a positive effect on the oxidation of ammonium and nitrites.
Molybdenum. Molybdenum has a positive effect on the oxidation of ammonium and nitrites.
Cobalt. Cobalt negatively affects the oxidation of nitrites to nitrates.
Copper. Copper negatively affects the oxidation of nitrites to nitrates.
Zinc. A zinc concentration starting from 0.48 mg/l has a negative effect on the oxidation of nitrites to nitrates.
Thus, elements and substances such as phosphorus, magnesium, calcium, and potassium, as well as especially phosphates, play a major positive role in normal nitrification in the aquarium.
Filter problems. Power dropped, External filter leaking, Filter is noisy.
Water filtration materials. Mechanical masses, Biomaterial. 
