The best/hardest lesson you've learned?

That life is to be lived and enjoyed and to not waste time on useless emotions like anger. Being nice to others, even if a stranger, can be rewarding.

That the life you know can come to a crashing halt in a split second.
What you said above was just beautiful and SO TRUE...
 
Cycling a tank properly is important. Learned that the hard way.
 
Most important fish related thing might be to always transfer stock over a bucket.. or a 2nd container.

I learned that a full bag of fish will lose its shape, and any kind of support when it is cut open.. I dropped a whole bag of cardinal tetras on the floor once while trying to cut the bag open. Now I usually put the bag in some sort of container just in case.
 
AquariaCentral.com