I wish I had known that my betta needed more than a tiny unfiltered, unheated bowl (my first fish, I knew next to nothing)
My greatest mistake was not quarantining my Electric Blue Acara long enough, and now my whole tank has ich
Edit: I lied, my greatest mistake is leaving my fish food anywhere my dog can reach! I can treat ich, I can't get back that $7 bottle of worms and those 5 entire cups of flake food
Answer to the third question is water changes. I used to have a tiny cup and a one gallon pitcher, and that was how I did my water changes on my ten gallon. I finally decided to just pay for the $5 siphon