I have a Top Fin 25w heater in my 5 gallon tank. See, there aren't temperature indictators near the knob. It just says from 68 to 93 degrees. So, what I usually do when the water is chilly at about 73F or less, I crank the heater up all the way. It takes about an hour and it will be 78F or so. Then it will drop over time, so I have to keep turning it on and off.
Do I just leave it at the point where I want it and never turn it off? I'm afraid my fish will go into shock from the changing temperatures.
Well, they are automatic, so doing that just makes the temperatures fluctuate.
Set it at 80 and leave it on/plugged in. If it gets cooler than 80, even by a degree, it will turn on and heat it up until it is 80. If it is at 80 or higher, it turns off automatically :) It basically works the way the AC and heating in your house works. Set it at the temp you want the house at, and it'll keep it there.
Just make sure it stays fully submerged all the time. There should be a little light in the front that turns either green or red. Green means the water temperature is good, red means it is cool and is in the process of heating it. However, just to be on the safe side (should the heater break), it might be a good idea to invest in a 1 dollar glass tube aquarium thermometor that can stick to the side and tell you approximately what the tank temp is.
One more thing. On the side, there is a thermometer. It reads 68, 72, 75, 78, etc.
When you turn the knob, you can see orange rising up behind the numbers. I turn it up to 78, but the "ON" light doesn't come on untill you turn it up to where its reading 84 degrees?
hmm, that's wierd. Try setting it around 78 and leaving it (keep an eye on it) and see if the on is working. Over the next few hours (if you have a thermometor available), just take random notes of what the real water temperature is. If it is working, it should stay within 1 to 2 degrees of the temperature you set it at.
I could have my colors mixed up... green might mean it's heating. I was under the impression we had the same heater, but it sounds like yours is slightly different. Do you have the instructions with you or available? It should specify which colors mean what. :)
How much water flow is in the tank? A lot of times what I've found with tanks with little to no water flow is that the water around the heater will heat up, causing the thermostat to turn off and creating cold spots around the rest of the tank. When you turn the thermostat setting up above the current temperature of the water around the heater, you'll see it turn on, but soon the water around the heater will just get even warmer and it will turn off again. I've had this happen several times when I was just starting out. The easiest way to fix this on a filtered tank is to position the heater near the intake tube. That way the water is constantly moving around the heater and the thermostat will work properly.
If you try this and it still doesn't keep a constant temp it may be time to replace the heater altogether. Some brands definitely are better than others ;)