My biggest mistake was with a new 75 gal freshwater tank. It was in the 70s and "good" water had to be bought in milk style gal jugs. Wasn't going to do that so the plan was to fill it with a hose and dose it, adding several days of filtering it before adding fish. Brought the hose in and started filling. Got it about 3/4 full when the wife called saying that she had been in a car wreck, and was at the hospital. I grabbed my coat and out the door I went. We were there for about 5 hours, and then I was up all night, the rug was ruined and, needless to say, the wife not happy.