Hi - new member here.
Thought I'd add my lessons...
1) If there are small children in the house, keep the test kits and drugs locked up.
2) Don't let *anybody* feed your fish without asking
3) Don't mix male and female jewelfish
4) Putting interesting and attractive ornaments in an aquarium will invite little fingers to play with them
5) A guppy is not a sheep, and a net is not a sheepdog - leave the fish in peace
(I was 12 at the time)
6) Don't get distracted while filling the tank (at least the carpet doesn't seem to smell funny)
A useful lesson though - provided you can keep small fingers from messing with them, Legos and Megabloks do appear to be aquarium safe (and the megablok man o' war ship from the dragons series looks really cool)