My biggest mistake wasn't really my mistake exactly. I simply listened to the dude at the LFS when he said a green brittle star was reef safe. Well by reef safe he meant it wouldn't eat corals, what I THOUGHT he meant was that it would co-exist without harming anything in my tank (Normally what is meant). Several fish later I finally caught the bastard in the act and then had to take apart large portions of my rock work to get the big sucker out.

Lesson learned: People at the LFS aren't necessarily speaking in the same context as you so do your own research and trust nothing unless you've verified it with a valued source.

About 6 years ago, I attempted to QT my fish with ich in hyposalinity (1.009).

I dropped them from 1.026 to 1.009 properly (3 points per hour).

I noticed the fish not doing very well once I got to 1.009 so I quickly added some salt to the water and they bounced back. The following day I got some proper calibration fluid and I was off my 3 points. What I thought was 1.009 was actually 1.006.

Lesson learned. :mrgreen:

I think this is the biggest mistake we all make especially when starting out. It took me a couple of years to wise up and stop listening to guys at LFS

Don't set a 400w fixture on your center brace.

http://i200.photobucket.com/albums/a...d/IMG_1987.jpg

OK mine looks like the 3rd worst mistake after the cracked centre brace and the light fixture dumped in the tank. Now I don't feel quite so stupid.

mistakes
 

putting a helfrichi gobie in my tank.my male bird wrasse had a $140 lunch in less than 10 seconds

O ya I forgot one I put my 3 250 watt mh fixtures on the glass had a fish splash the glass and broke

salt + clam..
 

got really lazy one day,,, added salt directly to my tank,,, straight into a clam..... :(

Might as well post mine.
Years ago, I was doing a tank move and fished out all my fish to a 5 gallon pail. As the move was taking longer than expected, after some time I figured that the water must have cooled off too much, so I decided to place a heater to warm up the water a bit.
Even though I had the thermostat set to turn off when the water would heat up to the desired temperature, I "boiled" all my fish as I did not have a small circulating pump in the bucket.
It still pains me to think about those poor fish that day, even though it happened so many years ago :(

I was messing around in the sump and must of bumped the phosban reactor. Unfortunately I didn't realize I had and the hose came loose causing water from the sump to leak out onto the floor. Got home from work the next day to a soaked basement carpet. At first I thought the sump had cracked and was leaking until Doug found the hose disconnected.