What's the biggest mistake you've ever made?

Most of the common ones have been said. I've had six goldfish in a 6G (gallon per fish!), no cycling, emptying out and scrubbing, then refilling, being lazy when feeding and not sticking my fingers in the pot, promptly throwing half the food in. The airpipe thing happens quite often...hmmm....but I've installed the one-way things now.

The worst thing I've done is with the Python. I had it sucking and it started coming off the tap. I didn't stop it sucking and ran out to stop the tap...my favourite fish got sucked in, an oranda goldfish, and pretty badly ripped up. I had to perform fishy surgery...but he is alive and well now. And I don't even leave it near the top of my tank when I'm out the room.

I also forgot to put the lid on my frog tank after a WC and two of them mysteriously disappeared...I have yet to find them, but my room is getting recarpeted in September so I will have to then, I suppose.

My most sickening mistake was not finding out how much this all cost before I got into it...
 
Other than starting this hobby back up after a nearly 25 year hiatus, I would have to say trusting myself was the worst thing I could have done. Knowing full well I have memory and ADD problems I walked away while doing a water change. Luckily there were low pocket of gravel that the fish hunkered down in that held water and then to top it off, I walked away while filling a 100 gallon tank. After it filled up the sump, the water decided that our hardware floors were thirsty needed to share in the bath. Took me 3-4 hours to clean up the floors and almost a week to dry them out without warping them. Needless to say, I sit there and watch tanks empty and fill. I don't leave unless I turn off the siphon first.
 
Overstocking was my #1 mistake. Not quarantining is #2.
 
I recently was gravel vacuuming my tank. One of my zebra danios kept dancing with the hose, but I shooed him off.

As I was emptying the bucket of yuck down the bathtub drain I saw a splash of fish. I screamed, threw my hands into the tub and grabbed him out. My husband ran into the bathroom asking what was wrong. I shoved him out of the way screaming, "Move! Go! Move!" The lucky little zebra lived to tell all his buddies about his almost Nemo-like escape and I now check the bucket before I dump.
 
I have now, um, bathed our kitchen floor with the Python 3 times. :o Twice because the sink wasn't draining right and I was around the corner in the office and couldn't see, and once because the part that connects the hose to the adapter came unscrewed, and the hose came flying off when I closed the nozzle when refilling, causing a stream of water to shoot into the kitchen. :o Luckily we have almost all tile.
 
while doing a water change on my largest tanks I decided to not do the other tanks that night because I was tired and exhausted.

well at the last second I decided to do the other tanks and when I was done I had completely forgotten water treatment

someone else walked by all the tanks and said did you put conditioner in here? the fish look funny, ran over and all of the fish were listing to their sides on the bottom and there was already a casualty. luckily I had the big tank treated so I was able to take water from there to put em in real quick without stressing them too much

that chlorine works fast I'll tell you it was only about 5 minutes of them being in there.
 
My first tank I had a few years back back when i knew nothing lol I am just learning now. I Save a big ole oscar. he was in a 10 gal tank I think it was you could not even see the fish it was so filled with crud he took up most of the tank.. i brought him home and put him in a nice 55 gallon had em for bout a year when I decided to get a pretty cichlid...she was a mean mean girl. she had some fry and that big ole oscar thought yum dinner ! well she ran him out of the tank. the shame of this story is I could not react in time before my dog timber ( timber wolf) thought yum ocsar for dinner :( that broke my heart.that oscar had no fear of me i could reach into the tank to handle him by hand others had tried and the never got close.
 
I used to (and i wasn't so little at the time) every month replace my filter pad with a new one, take the fish out, all the ornaments, the gravel, EVERYTHING. Then i would put 2 capfulls of CLOROX into the tank because I was "disinfecting" it and I would also wash my ornaments and gravel in the CLOROX and rinsing everything off and putting it together again. As you can imagine I was starting a whole new cycle (i didn't even know it existed), every month and killed A LOT of fish until I finally decided to read on fishkeeping.
 
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