Listening to your LFS about ANYTHING.
1) "Oh these are the best way to go, just suction cup them to the inside of the tank and they'll constantly monitor your pH and ammonia levels." Needless to say I wasted 10 bucks.
2)"Just bring in a water sample and we'll test it. If anythings wrong we can get [sell] you what you need to fix it" No, bad LFS. NO CHEMICALS TO FIX THINGS. (Besides conditioner of course) PWC's!
3)"Oh yes, a 20 gallon tank is plenty for that fish" Umm, no ma'am it's not. It needs 50 gallons at least, thank you very much.
4)"Let it run for 24-48 hours before putting fish in. After that it's all enjoying the fish, works done" It doesn't work like that.. AT ALL.
5)"You know, it's not That crucial to test your water unless everything keeps dying." ??????
And yes, every single one of those was said to me, and I believed it all till I found AA, which have saved my fish, and me. I'm sure there's more those are just the ones that stand out.
Lesson: Ask before buying, and don't buy uninformed. Being informed does NOT mean asking someone at your LFS, even when they say they're the "Aquatics Expert". Which is who told me all of that stuff.