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 ^.^
DO YOUR RESEARCH!