also it would be neat if the heater could be set to a certain temperature so that it would properly respond to room temperature fluctuations (keeping in a high school biology lab). I've never had to deal with heaters so they're not familiar turf.
i had a cheap heater in my fluval flora. it never worked, would only work if i turned it on to 92f which everything would die in. i bought an eheim jager 25 w and my water is always at the temp i want it to be at. very reliable and easily calibrated. you do have to calibrate it to make it work well. keep in mind it is a pretty big heater for only 25w.