When I was a kid, I use to do water changes on a ten gallon by putting all my tetra's, catfish, barbs and whatever else I had in a big gulp cup, and cleaning the entire tank in the shower. I knew nothing about O2 in water (I thought fish breathed water) or tropicals needing heater or dechlorinator. One time I put them in the cup with ice cold tap water(i usually had the sense to use room temp water but didn't for some reason this time) Probably took me a good hour to clean the entire tank, and when I came back they were all dead in the cup, with bloodshot eyes. I was sad and dumped them into the tank and went to tell my ma, when we came back in the room minutes later ALL the fish were alive.