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

I put my krib fry in an uncycled grow out tank and didn't change or test the water every day. One morning when I checked on them, they were all dead except one. The nitrite test results were off the chart.
 
about 7 years ago i got a ten gallon tank from a friend and proceded to stock it with five comets.
then when they started showing streaking in their fins about a week later, i went to the pet shop and they told me the fish had septicimia, so i was medicating the crap out out of them for no reason while they died of ammonia poisoning.

glad i know what cycling is now.
 
I once spent a month growing out about 100 Xystichromis phytophagus in a tub outside, then moved them back to an inside tank without acclimating. They all died from ph shock in 10 minutes. *kicks self*
 
My biggest mistake is probably keeping 2 Guppies and 5 C. habrosus in a 2.5 gallon uncycled tank during vacation. Although, I was doing some water changes ones a day. So, Im not sure what exactly happened. Im never keeping fish over vacation again.
 
Putting fish I just bought from LFS directly into main tank without QT first. Live and learn I lost fish by doing that. I am in the process of purchasing a QT will never add new fish from the LFS directly to my main tank ever again. Makes sense to have the QT even LFS owner agrees is a good idea!
 
Ugh. My latest idiot mistake was overdosing the plant fertilizer. I lost a couple of diamond and rummynose tetras. That was *thankfully* a while ago.
 
My biggest mistake.

Hi I'm new here. My biggest mistake was keeping 2 bettas (one male one female) in a divided tank that was less than 1 GALLON!:duh: and the tank was not aerated (spelling?) or filtered, I didn't even change the water that often:shakehead: needless to say they both died quickly:tombstone::cry::cry::cry:
 
Thank god I havent had any huge huge mistakes. Ill have to be careful with that canister filter of mine.
My worst would be when I got my original fish tank.
I set it up, read instructions and filled it with fish 2 days later. Fish were fine, until I found out that one of I fish I had, a blue gourami, was a big bully and 5 gallons wasnt enough for him. I euthanized him and a gold gourami shortly after, and was stuck with 2 dwarf gourami and a platy. The platy strangely died... and the dwarves started to nip at eachother. Needless to say, I no longer have any gourami or paltys or guppies.
Another mistake was buying my dragon goby and putting him in my 5 gallon. It was only for a few days, but i still feel bad for it. As well, not checking my levels... that is soo bad!
My goby is now in a 30 gallon and is going to go into a 135 ^.^

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