My greatest lesson cost me nicely. I was growing up some valuable fry fish when I went and changed their heater for no apparent reason. Their tank was 76 degrees and I've always like keeping fish in the 80's so I swapped out the heater in the tank with one that was lying around that was higher in wattage. I did it at night and when I woke in the morning. FISH SOUP! All the fish were cooked alive. Nearly $3000 worth of fish. The moral of this story is, When it isn't broke, don't f*ck with it!