If one wet pair of swim trunks can contaminate a 20,000 gallon pool with chlorine in it. I cant I magine how small of an amount it would take to contaminate a aquarium. uv light salt and a strong imunne system will lead to less chances of getting fish even getting sick. If it happens the fish would have a better time fighting it of any thing off that is If anthing was in the water. What is easyer to treat 100 gallon tank or a 100 1 gallon tanks. If one fails its easy to medicate one tank rather thank the whole 100 gallons being comprimised. That is what I was stating. Water quality nutrition is prevention is what I was trying to not ramble through. Getting back to mold algea and bacteria, they break down waste and turn things into other chemicls toxic chemicls. Some molds and algees produce gases chemistry is life it finds a way to react. No disrespct all I was trying to say. Medince are good but I feel they are the run to and are over used and get less effective ofer time. The they have to produce super new chemicles. But if the fishies whould spend money on making fish heather then aquaruim diesies would not be so strong now What I was trying to say with out the rambling.