Sick Tank Syndrome

Hello, so I have been researching into Sick tank syndrome etc, https://aquariumscience.org/index.php/18-7-sick-tank-syndrome/ (the article) so I have a few questions, if sick tank syndrome is semi correlated due to not enough bio filtration, etc, how do you know you have enough bio filtration if that makes sense. There’s really no calculator online telling you what type of bio filtration you need for a healthy tank.

The author writes: “ a newcomer with problems with disease in the fish by asking “what are your water parameters?” They should be asking “how clear is your water?”

My question is How are you able to tell if the water is “crystal clear” when you have fish that like to burrow in the sand or even tannins in the water? Couldn’t the fish that moves the sand around cause some sort of substrate particles floating around especially if you have fine sand?

Next is about fish food, the author writes: “Note the “45% protein” level is completely arbitrary. In truth there is a continuum between 30% and 60%. 42% won’t be “ideal” but it also won’t be that bad. The key is the clarity of the water. Is the 42% protein food giving crystal clear water?”

My question is do I only feed the fish extremely high protein base food? How does that work for algae eaters?

The author goes onto saying: “Dull” or “slightly cloudy” water is unhealthy water, even if ammonia, nitrite, and nitrate are all zero. “Dull” or “slightly cloudy” water is water which is filled with bacteria and a lot of other organisms. These bacteria negatively impact the immune system of a fish, and this indirectly allows disease organisms to attack the fish.”

So now I’m extremely worried about my fish, as I don’t have good eyesight and I have Kuhli loaches which are sensitive fish and act like complete chaos 😅. If we can’t see or even test for these harmful micro bacteria how do we control it? Cause I read that doing to many water changes can harm the tank or throw it off balance

So when doing a water change should we add Beneficial bacteria supplement to help the tank even if it’s cycled? Or should you just add stability into the tank during a water change?