Ive had fish my whole life and the biggest and most costly mistake i have ever made is buying a gravel vacuum for Christmas. Here's why.
I recently went out to my mums where my 2 monsters r to take some pics of them to show on here and decided it would be a good chance to test my gravel vacuum out on a bigger tank after trying it on my small 4.5 gallon tank where i managed to suck up every fish through the tube and all of the water down to gravel level into a bucket in less then 5 seconds. (I didn't think it would go that quick.)
So i got there and go into my parents computer room which is the biggest room in the house with the most expensive things and fish tanks lining the walls and floor. Went to use it on my tank and realized it was 30cm short of reaching the door. I thought id be really smart and connect the yard hose to the gravel vacuum. After a lot of effort trying to get the hose that came with it off i finally got it and connected the yard hose.
The hose fit perfect and i started removing the water and stuff from the gravel and all went well. I was so happy with myself thinking "oh aren't i smart bet no one thought of doing that before did they

" when another brilliant idea came to mind.. instead of disconnecting the hose to add more water to my tank ill just leave it on and get my bro to hold it for me while i turn on the tap outside.
So i drag my brother away from his ps2 and say to him
"hold this hose for me in the tank while i go turn it on and yell out when it gets to here." *point to a mark on the tank* he agreed and i went out side to the tap an turned it on (full blast of course because I'm hardcore like that and my fish like the air bubbles the hose makes when the waters going in fast... no i dont and have never declorenated my water at all in the year ive had them and my fish are perfectly fine and healthy) anyway i hear my bro go "um its making a bubbling noise." so i yell back to him "yea dont worry just tell me when its gets to the mark." about 2 seconds pass and i hear him go "Um Yillant?" and now im like god whats wrong ill go hold the hose myself and i walk to the door open the screen and say "what now?" he looks at me with the most calm expression on his face and points to the floor... the walls... the curtains.. the ceiling.. the chairs.. then to the hose which is running full blast spinning around the floor and im like ahhhhhhhhhhh run to the tap and turn it off and back inside and me n him r just watching the water drip from the roof to the cm or so deep puddle of water covering the entire floor... and he goes to me "Youre so dead." and im like "Im dead?!?? Youre the one who doesnt know how to hold a hose! Its not that hard u point it into the tank and hold it there you idiot!" and he comes right back at me "Im the idiot?!?? Youre the idiot who connected the hose to something with a valve you moron!" and suddenly i realise hes right "oops" ...... basicly the only dry things in the room were him and my dad who was looking out the the other door absolutely oblivious to what had happened
.... 2 $1400 cameras... 1 $1500 computer.. a $2000+ computer... Several different UHF radios and stuff worth a few thousand.. powersuply for them.. a guitar amp thing.. all soaked
many many hours of towelling.. mopping the ceiling and drying things with a hairdryer...
Lesson Learned: Gravel Vacumms have valves!