1. Old water. It was widely believed that old, unchanged water, pale yellow for some reason, had semi-magical properties for avoiding disease. That one got flushed early.
2. The Balanced Aquarium. It was to be your goal to create a no water change aquarium by creating a perfect balance of light, nutrients, plants, waste, feeding, stocking. Then somehow that balance would maintain itself in your tank. That still comes back, usually sold by white haired men with suspenders (for some reason) on youtube. It still begs the question of how you would maintain perfection unchanged. I was a sucker for that one in my teens.
These are the ones I used to battle with dad over! He was keeping and selling fish in the 50s-80s, but even then, he was really the bird person, while my mum managed the aquatics. Even after the business, he kept a personal tank throughout my childhood in the 80s - 90s and later.
What I remember, he rarely gravel cleaned or did water changes, would just have a big tank clean when it got ugly or every six months or so, otherwise it just got topped up. But it always had live plants and generally would be what I'd now consider understocked. I remember really enjoying gravel vac-ing when I was a kid, because it seemed kinda magical to me, lol. Always had charcoal in the filter too.
Decades later when both parents needed care and I moved in to help, I took over tank maintenance since he was struggling with caring for his tank by then and had added a lot more livebearers,
I did a lot of research and ended up getting into fish myself, but I was learning more new school methods, like here. I got an API freshwater test kit, and the nitrAtes were so high, they were unreadable. It took many small water changes (to avoid old tank syndrome) just to bring them into readable levels. He didn't like and distrusted all the water changing! But gradually accepted that we just had different ways of doing things (although I'd often sneak in water changes while he napped or at night, so we wouldn't argue over it!
But there was a definitely old school/new school clash that I experienced with him, and have seen play out on the forum too, and I find it pretty fascinating. I've kept some of the remaining fish care books they had, but I know they got rid of a lot that I wish they'd kept! Not sure when the hobby seems to switch from old water - new water, but there was definitely a change in fishkeeping methods.
For those old school keepers out there, was it normal to turn off the filter to "rest the motor"? Dad used to turn the filter off randomly for a few days and said the motor needed resting, until the tank would start to look cloudy, then he'd turn it back on again. Sadly had more than one tank crash as a result of this habit.