Your Biggest Newb Mistake?

Buying and keeping 4 (four, not a typo) Green Spotted Puffers (tetraodon nigroviridis)in a 10-gallon FW tank. But that was 40 years ago... I have learned a bit since then. The amazing thing is they all lived in that situation for over a year when they were moved up in the world, and to better water.
 
I set up my first 55, and decided I needed to have airstones for oxygenation. I was already about halfway through a fishy cycle with two giant Danio's. I installed the stones and hoses but my old air pump didn't work so I left the hoses hanging with the intent of getting a new pump on my way home from work. What I didn't know was that water current can and will push water upward through a 1/4 line and start a siphon. There was still about an inch of water left in the tank when I got home so the danio's didn't die but cleaning 40 + gallons of fishy water out of the carpet was no fun at all.
 
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