I feel bad for the fish I kept going back decades, but…honestly….it was done that way back then…..UGFs, no WCs to speak of…just topping off, and only minimal water movement/filter turnover. And I really feel bad because I am a trained biologist as was my father and we should have known better. When I learned of the merits of better water management with fish, I forever changed my approach both for higher success and, more importantly, a better life for the fish in my charge.
This FF stuff is simply a no brainer

If in doubt, take a moth larva, put it in a tupperware container with some food and change his food and clean it out daily. Then take a second Tupperware with the same species larva and repeat this but merely add food with no cleaning out. After just a few days, the second one will start to smell, get dirty, moldy, and its health will deteriorate. It makes 0 sense to keep fish in a closed container (aquarium holding water being equal to a closed Tupperware container holding air) without changing it with FRESH clean water. Even people living in filth, sadly, have the same problems as such conditions lower both the quality of life and, imo, the duration of life.
I agree that showing/linking FF’s vid is unwise as there are people who will blindly follow such “stuff” because it looks and sounds like he knows what he talking about. ******99.9% of fish in the wild do not live in the same exact water. They swim to different areas having much larger water areas to swim in than a fish tank, it rains, there’s water runoff….or….water itself moves along and is replaced with new water.
Lastly…..if questions/doubts still abound, go hug a dog kept in filthy conditions (his environment not having been cleaned/fecal and urine material) and compare that to hugging a dog kept in a clean home.
Finally……think about that animal (fish) that is TOTALLY under your care control for its existence…..why not make it as good as possible?

P.s. sorry about the rant….I held off for as long as I could with this “FF stuff”.