So, I'm very confused....
most of the reviews of the small mini heaters (7.5watt) are bad saying that they cook their fish.
The TopFin 25w adjustible is waaaay to big for my 2.5g minibow tank
Size-wise the aqueon 10w would be good, but, again, the reviews scare me.
The problem with reviews is that a lot of times people complain about problems that are: a) inherent to the design (so they are using the device where it should not be used), b) operating the device under parameters that it is not designed for, c) not properly taking care of the device, d) the device does not satisfy their fantasy of a miracle doer and solve-all and e) due to the device being faulty (by manufacturer, delivery, etc...) and they are too damn lazy to return it or ask for a refund. It is however possible that the product is not that good. It's the same concept as deciding where to go on a vacation.
Personally I have never had any problems with the Aqueon 10W Mini. It has always maintained the correct temperature needed for my betta in my 2.5 gallon glass tank. It even keeps the water at the proper temperature when I "air out" my room (currently winter where I live with temperatures on average of -10 to -20 celsius). I use the heater as directed by the manufacturer on the instructions provided with the heater; that means that I never let the water level fall below the minimum line, I unplug the heater whenever I put my hands in the tank (for whatever reasons such as decor rearrangement, something fell in, etc...) and I unplug it, clean it and visually inspect it on occasions when I do a full tank clean.
Here is a picture of my aquarium (2.5 gallons) with the Aqueon 10W Mini Heater to show the space that it takes up (~2.5cm width, ~12cm height, ~1cm width).
You can place heaters horizontal if you have to. I have used my 25w in my 1 gallon qt tank before and had no issues... Plus it is adjustable and reliable, so that is worth the tight fit in my book. Plus if you decide to upsize eventually you won't have to get another heater for the bigger tank.