My worst mistake? Not putting my tank up high enough. My son poured soda into my tank!! Thank goodness there were no fish in there. But, I had to start ALL over. Could've been worse. In one of my freshwater tanks, he put (all at one time!) a pair of his "big boy underwear", McDonald's french fries, a bowl of cucumbers (bowl and all) and a used dryer sheet. Luckily, my hearty little goldfish fared well. They even enjoyed the cucumbers quite a bit. So, maybe, my mistake is just HAVING a tank period with a three year old!!
BEAT THAT BABY!!