I have a one gallon hex, with
UGF. I put the smallest heater i could find, that had an actual thermostat (25w) so the temp seems to stay within a few degrees.
Interestingly enough, my entire Aquarium hobby started with this tank, I bought it so I could keep a Betta on my desk at Work. He lived a good long time in there, and when he eventually died, I was given some poor advice at my
LFS, and bought a dwarf Gourami. The DG lived in there for about 18 months, but he really was getting too big for that Tank (probably he was always too big for that tank). Which was the excuse I used to buy my 55, and now that same DG is the King of his own kingdom in the Main tank.
So now I still have this 1gallon Hex. It sees various duty, for one thing I can tear it down, clean it, and set it back up with gravel and water from the main tank. So now I have used it for
QT, as a Hospital (the 1 gallon makes dosing meds easy, and the size and ease of cleaning help there too). Right now, I am using for a couple baby Rainbow Fish I an raising.
They started in a net breeder in the Main tank, but I had probs maintaining water quality within the net. So now they live in the 1 gallon hex. There are two Rainbows under 1/2 inch each, and two small Ghost (Grass) shrimps. The shrimp makes it a bit easier to feed the Babies, as they eat any thing that fall past the rainbows. The rainbows only eat food thats falling, not on the bottom, and not on the top.
Very soon, they will outgrow this tank, and be big enough to survive the main tank. So all in all, I'm happy with my one gallon. Mostly cuz it is easy to set up, tear down, and clean. It works well in support of my other tanks.