The filter cycle would have been good. I learnt that about 6 years into the hobby when I started keeping marine fish. Back in the day the marine books had the info and the freshwater fish books didn't.
Then diseases would have been good to know about and alternative treatments like salt and heat. I killed heaps of fish by overdosing medications because the shops told me the tank held X gallons of water, when in fact they held considerably less.
One of the things I did when I started working in the pet industry was write down how to work out the volume of water on the back of the shop cards when customers where treating fish. Just so they didn't overdose and wipe out the tank.
Finally, I would have liked to know more about fish diets. I started keeping rainbowfish before they were popular and there was nothing known about them except they came from Australia. If I had known they were primarily vegetarian and sensitive to chemicals, I wouldn't have lost anywhere near as many as I did.
Yep, back when I was a wee lad, we would fill up the 10 gal with UGF and carbon filters in the outflow tubes and add a bunch of fish. Add water whenever needed, remove dead fish, replace fish, when all or most were dead we would wash the tank, sometimes with dish soap to be sure it was clean. Repeat ...