Aqueon Aquatic Flat Heaters 7.5 Watts Black 4.75" x 1.75" x 8"

Was skeptical after reading bad reviews but I am happy with this heater. I have it in a 3 gallon tank. FYI the instructions say to soak it for 30 min before plugging it in the first time so that may be why some of them melt...,

Too big for my one gallon tank but it does what it’s supposed to. It packs a powerful heater punch. Great for big tanks of 2 gallons or more

This fits perfectly in my 3.5 gallon tank. It clamps onto the inside and lays neatly against the glass. It's never in the way, and doesn't bother the fish. The water stays at a temperature between 75 and 79 since I've been using this. I would purchase this again if I had it to do all over again.

This submersible automatic heater works great. I use it in my Fluval spec iii. I keep it in the back right side where the water flows right before the filter current pushes it out. It is of course warmer back there by one degree since it is trapped in a small area with the filter pump. The water where the fish swims stays at a constant 79 degrees. I never have to worry about this heater, it does it’s job and keeps my fish happy and healthy. I highly recommend this for 2.5-5 gallons.

This is a grand slam for chicken waterer buckets! Keeps it just warm enough for the water in the buckets not to freeze and the chickens are not bothered by drinking warm water. I use the drip nozzles. Just keep bucket and heater (rubber disc) clean from slime and you are good to go!