Oi....
As a first-time owner of a large tank with a sump (which turned out to be a lemon, more on that later.)
1. Left the return too far below the waterline and shut off the return pump. Darned thing backsiphoned, flooding the sump and three or four rooms with copious amounts of saltwater before I figured out what was wrong. Biggest cleanup I've ever had and both my mom and I have fibromyalgia. ..it was just the two of us cleaning up >.<
2. Got burned on a $400 used tank because I failed to check the middle brace up top and didn't know what causes crazing. At least I got my money's worth of rock and sand out of it...which leached a ton of phosphate even after soaking and rinsing in clean water...
3. When the bad tank was filled, it bowed badly, but I didn't want to waste all that saltwater, so (without testing it...bad move) I used it for a couple weeks to change the water in my 20g. Ended up with the worst greenwater bloom you've ever seen in your life.
4. Let me just put it this way - I wish someone had warned me about the incredible leaps an anthias can take before I brought one home. Both leapt at least a good two feet out of the bottom of their acclimation bucket and onto the floor. I was sick with worry for a good week after that. What? My dream fish had just hit the floor - hard.
5. I've overflowed my RO jugs more times that I can count...and this is me awake. >.<
6. Failed to include a thermometer in my quarantine, accidentally killing a gramma when the heater accidentally got set to 66F for two or three days.
7. Used lanthanum chloride without researching it (or dosing properly, as I learned later)...not sure if it killed my fish or the concurrent dosing of two denitrifying bacteria supplements, or both, but I woke up to all three of my beautiful babies dead.
8. Poorly packing corals for a move, resulting in the death of two of them.
9. Getting into this hobby in the first place. XD Mom calls me a fishy crackhead. I'm obsessed.
10. Putting three electric blue damsels in the same 29g tank.
11. Left my frozen fish food out at least three times...I hate doing that because it lowers the quality of the food and, on the case of one as poorly packaged as
nutramar ova can be, makes quite a mess. Quick tip - if you buy the ova, put it in a plastic ziploc.