Emmy

my tank's been set up for almost a week and i can't get the thermometer and the heater to agree. i have a 20gal tank and a 100w heater, it is a rena cal top light excel heater. at first a 50w was recommended so that's what i got (following advice of LFS guy), got home, ran it for a day and noticed the light was constantly on, so i had a closer look at the pkging and realized i need a 100w. so i exchanged it right away. i allowed the water temp to drop to 74 (according to the thermometer). i set the heater to 76 and put it in the water (fully submersed and verticle, about 4 inches to the side of the filter). the temp has climbed to the top line of the green area on my simple glass thermometer which is 80f... i am pretty sure. and that is where it's staying. what could be wrong, any ideas why the setting doesn't agree with the thermometer, has anyone experienced any weirdness with this type of heater, i've read nothing but rave reviews about these heaters! i don't know what to think, maybe because when i put the heater in the tank to rest (unplugged) the water was already warm so it messed up the setting? the guy at the lfs told me to move the thermometer away from the heater and see what happens. duh, imagine the temp being the same on the other side of the tank! perhaps i'm missing something obvious? help please.

i thought i'd go a day without asking a question but no. for a newbie i guess my name will become well known on this site with daily questions... sorry to ask so many but you guys seem to know so much more than the bloody stores.

cerianthus

Cheap and mass produced thermometer is not the best one to test if heater is working properly. It could also be the heater where calibration done by Manuf. is slightly off.

Few methods to see whats working and not:

1. Even if you have heater set at 76, if room temp goes higher than 76, your tank will rise in temp even though light on the heater is OFF. If hightest temp in your room is 80, trying setting at 80 to see if it maintains 80. If heater is on and goes above 80, maybe calibration is slightly off. Then use judgement call to set to desired temp or exchange for new one.
If temp is maintained at 80, leave it at that setting so that to minimize fluctuation of temp which is not good for fish (Ick can start due to fluctuation)
I've used and still use same Heaters w/o major problems.

2. To test your glass thermometer, fill any container w/ about 76 degree tap water according to your glass thermometer. Then use Thermometer you use for yourself (digital at drug store, about $10). This way you can find out if thermometer is off. You can even buy digital thermometer for fishtank for about $10-15 on online stores.

Hope this helped you a bit in relieving some frustration you may have experienced.

Oh, one more thing, always leave the heater in the water for at least 30 minutes before plungging to allow the thermostat get used to exsiting tank water temp.