Like the two awesome posters said above....you gotta keep that temp stable. My room temp is 65*, and my tanks stay at 78* sans the salt tank, which is higher for obvious reasons. 100w is plenty for a 20g, but it has to be submerged below the water line on the heater to work. I've had one explode in my old SA themed tank, and it was a god awful nightmare cleaning it up. I think I have a post on it here somewhere...hmm. Always unplug it when you are doing a water change. Make sure it's not on "always on" or however on your power strip. If it is not plugged in, it will not heat up. Another good trick is to check the water in the tank versus the water you are adding to thank with clean hands. 20g is a small tank, and water temperature can swing quite rapidly with water changes. A bit of a ramble, it's been a long day.
cheers.
. The thing is, bacteria are sensitive to shifts in temperature. If you cycle at 86 you'll get bacteria that like 86 degrees and then you'll kill them off and have another cycle when you lower the temperature. Granted you won't kill them all off, but you won't have a full load of BB when you start to add fish and will get ammonia spikes and mini cycles. You're much better off cycling the tank at the temperature you want for the fish you're going to get. Regardless of temperature cycling will take as long as it does. For some people its a 4-6 weeks, less sometimes if you get a seeded filter. For others its 3 to 4 months and I have some friends who were trying to cycle a tank for 8 months and just gave up and did a fish in cycle.
