Originally Posted by Rohland
i am going to see what temperature my tank maintains without a heater.
Responding to your last two posts on the heater and heat.
Re the heater, most aquarists recommend a heater that is specified for a larger tank than the one you intend it for; the reason is that the larger the heater the less chance of it malfunctioning. I have myself had 2 or 3 50w heaters malfunction, but never a 150 or 200w. You don't say the wattage of your heater, but if it is intended for a 20g it should be fine for the 10g. Of course, it may not be working properly--what make is it? Some are more reliable than others. It sounds like one Ai have, on the lowest setting (which is marked 67F) it heats the tank consistently to 78F. I have it on my spare (no fish, just the plants) and it is consistent, but I don't trust it with a tank of fish.
On the room temperature, this works if your room can remain at the temp you want the tank, say 74F. No problem in the summer--although yesterday it would have been cooler. And in the winter you'd need to heat that room constantly. Fluctuating temperatures are stressful on fish and will lead to outbreaks of ich and other parasites. A diurnal fluctuation is OK because the fish are designed for it, but this varies by only a few degrees, say 71 at night and 74 in the day; most streams in Amazonas fluctuate by 1 or maybe 2 degrees celcius each day/night, but that's it for the entire year. I wouldn't risk the health of my fish going without a heater, not in our climate.
If you had a fishroom, with many tanks, it might pay to heat the room. Rhonda Wilson, who authors the monthly plant column in TFH, does this. But it means keeping the room at 78F which I personally would find warm and uncomfortable. Of course, she lives in a warmer climate so this is easier.