Oh man..the countless mistakes and money down the drain!
I remember when I first started out with a10gal tank from walmart...
-of course I used sink water! and stupid me I would just pour salt right into the tank! I bought about 5 damsels the first day..all of which died in about a week...
-Oh I cant forget the time I wanted shrimp..they were feeder shrimp (so like 12 for a dollar) i poured them in and they all burrowed in the sand...and I was thinking "this is really cool, I like these"..20min later..all dead! why? turned out to be fresh water shrimp..poor things were suffering trying to get away!
-A mistake I made much later on when I had a 55gal (fowlr) I switched from a regular hang over filter to canister filter...the first time came for me to change out media and wash out filter pads but I did it all with fresh water...Oh the heartache I had the next morning when I went to feed .... my niger and and copperband (which I had for a year or more) and some other fish were floating on top! Washing with freshwater had made the pH plunge way down and the only things that survived was a super human yellow tail blue damsel(hes gone thru so much) and goldstripe maroon clown (which i still have both today)

Gosh I feel so horrible all the mistakes Ive made!...today I am much more patient and make sure to research all I can!