Oh, my goodness, where to begin. The big one would probably be, "Don't start keeping fish until the internet is invented."
But seriously...When I was a kid, the available books pretty much said, "Plants are pretty, and they aren't supposed to just melt away and die like they do in your tank [but we aren't going to tell you how to prevent that]. Undergravel filters are pretty nifty, as are air stones. In fact, aeration and temperature are the only water parameters. OK, enough about all that silly stuff. Look, we have pictures of pretty fish!"
If someone had told me the following seven fun facts, it would have revolutionized my fish keeping and possibly my life. In approximate order of life-changingness:
1. The bacteria in the gravel are what keep the water clean. Scrubbing it clean and laying it out in the sun to dry when you clean the tank feels very satisfying, but it's counterproductive.
2. Water changes are a thing. Just replacing evaporated water isn't enough.
3. Aquatic plants need more light than what's coming in from the room.
4. Yes, ich treatment makes the water pretty, but adding it as a precaution after a water change is a bad idea.
5. #5 is doubly true when you have snails in the tank.
6. Small tanks mean small fish. Goldfish are too big and active to live in a 20 gallon tank. So are most native Wyoming fish. In fact, most fish need a LOT more space than you think.
7. Don't keep a channel catfish with anything that could possibly fit in its mouth.
8. It's amazing, the things that will fit in a channel catfish's mouth.