Over the years I've unfortunately had a few tank crash/or issues that resulted in things dying.. This are the ones that stand out.
1st) I had a Skimmerless 20L SPS tank that I was found of, but I using NSW for it, and it had a bacteria bloom.
2nd) when was I was moving. I had all my live stock in bins on the floor as I was tearing down, some how the heater probe got kicked/fell out of the bin so the temp probe was in the air and boiled the tank.
3rd) Was my 90g (1st time). At the time I was wasn't using RO/DI for top off. (never did in Socal wasn't an issue), turns out in mammoth we use a well system and sometimes we switch different wells either way.. one day my calcium was off the charts.. turns out they switched wells and the water now had calcium off the charts (and who knows what else).
4th) My 90g (2nd time..minicrash sigh).. JUNE BUGS! ...We didn't have any screens on the windows at my place and it backed the forest spring came and 100's of june bugs kept dying in my tank, polluted it didn't realize why things were dying. until I pulled my filter sock and saw it was full of 100's and 100's of june bugs. This in conjunction with a regal angle that had a taste for SPS (not LPS?!) had me giving up on sps in the 90g.
5th) I wouldn't call this a crash, but i did loose some stuff, I was still running a calcium reactor at the time I received 2 leaky c02 bottles (in a row) and my tank went 3-4 weeks w/out supplements. Unfortunately I can't get c02 locally so it took a week between each one to get a new bottle.. (that was the end of CX reactor for me)
6th) Years ago RC had some bad batches.. I was one of the unlucky ones that got one. (switched to hw after that)
7th) The annoying Salifert ALK test kit mess, around the same time or maybe a year or so after Salifert put out a new test kit that was very inaccurate there was alot of complaints and issues with it, I didn't crash the tank because of this, but I did loose some things before I stabilized my tank with ELOS alk kits (now using hanna and salifert again).
8th) my 500g ugh..this one sucked. Had a titanium heater pop on me (but only slightly). For MONTHS I was fighting trying to figure out why things were slowly dying (easy stuff to like sunset montis). I'd go over all my specs everything was fine, I'd check my equipment everything looked good, did all the typical stuff to no avail. Then one day I was pulling my arm out of the tank and with my hand still in my eblow touched my luminarc pendant.. I WENT FLYING (I was on a stool tall stand & 3' deep tank). Needless to say I checked every piece of equipment, until I finally found the bad heater. It took months for what was left to recover.
9th) Pure lazyiness.. on my 500g I had been running it for 3 years or so, I had gotten to a point where between my old business, and all the tanks I had.. I wasn't tankless for the good portion of a decade, I tried to do the set and forget thing (which so many can get away with) but I can't.. I ended up with nitrates off the charts. I took ALOT of water changes to fix everything, tank actually came back better then it was ... then 4-5 months later I sold it.. the guy who bought it off me lost everything in the move.
10th) This isn't really a crash but I did loose a couple pieces and, couldn't figure out why I was having issues with a few montis. It could have been much worse. (This was just the other month) I switched from hW to a RSCP all the specs I found for RSCP said ALK was 7.5.. Turns out it was supposed to be 12.2, the batch I got was 16++ Even with that I tried to use it small amounts not spike my alk (which I was able to do by the #'s). Something must have been bad with the salt things continued to get worse with every water change. It's now 16 days and 80 gallons of water changes since I switched back to IO, and everything is bouncing back strong.
Other "issues", or blunders I've had over they years.. Fish getting stuck in overflow drains, tanks cracking, lights falling into water, rio's going bad, bad test kits, refractormeters that won't hold calibration, trusting swing arms when I was new, fish eating sps, overstocking, plumbing coming undone and draining sumps, and of course the worst.. AEFW, and Monti eating nudis.. >_<