Going through nitrogen cycle on another tank...an African Cichlid tank. Levels are getting a bit high on the nitrates...ammonia and nitites are not super toxic at this point, but figured I'd do a 40% water change and supplement the tank with more TSS. I unplug the pump...and everything else except for a sponge filter...I thought. As I'm emptying the tank...I smell a slight burning smell. Check the new floor lamp I just got..the filters. the light....went outside the room and smelled around. It was coming from the room with all the fish tanks. Could NOT find the source. I;m getting ready to fill up the tank and the smell is getting a little stronger. I then look at the top of the tank and see smoke coming up from the back. I forgot....to unplug the heater. Aquatop Titanium 300w heater..that looks all scorched and nasty. It also burned the Universal Rocks background. It has what looks like a cigarette burn on the back of the tank. Stupid...wasn't concentrating....disposing of heater..just to be sure that there are no mechanical problems in the future. I also made up a big blue sheet of paper that says in bold type..."UNPLUG HEATER!" This is not the first time this has happened. I have multiple tanks and get a lot of practice. You'd think I'd figured it out by now.. NOPE!!!