Is the tank near a window where it gets direct sunlight? I doubt a 14 watt fluorescent bulb would add much heat. Get an accurate thermometer and place it near the tank, my guess is it will read the same as the tank. 80 degrees is OK for most tropicals, but it will go up as summer approches. Most heater settings are very inaccurate, yours is probably 8 degrees off, you could just durn it down to 68-70 in case of a real cold noght. Does the temp change if you leave the light off?

Even a small wattage fluorescent bulb will give off heat, and with only 7 gallons; yes, the bulb is heating the tank.

Fish in the wild experience temperature changes daily, so warmer in the day and cooler at night is not really a serious issue. It's sudden changes that shock them, and why they need to be floated in a bag before being placed in a new tank.

An unheated tank, regardless of size, will never get cooler than the ambient air temp, so if you keep your house in the mid 70's, the tank water will never fall below that and you will kill fish faster in overheated water, than by allowing them to live cooler at night.

The only perfectly accurate tank thermometers are battery powered, digital, instant reads, like the type used to take body tenmps, but they are very expensive. I have one that I use to test the accuracy of the others in my current 16 tank set-up and have found the old glass mercury thermometers to generally be more accurate than the stick on digital types.

Thanks for the replies. I got a glass thermometer and it's giving me the same reading as the liquid crystal one, so at least I know that it's accurate.

When my heater was in there, it was on the lowest setting (68 degrees) and the tank was still getting up to 85 degrees during the day. Do you think I should ask for a refund? seems like a broken heater to me.

With the heater removed, my tank temperature increases slowly during the day with the light on, peaks between 75 and 80, and drops slowly to 70 degrees at night with the light off. Do you think this is OK for tropical fish? Or should I get a timer for my heater so that it only comes on for a few hours at night? (I'm just concerned that if the water starts out warmer in the morning when the light comes on, then it will end up higher than 80 degrees by the end of the day.)

or maybe should I set my timer so that the light is only on for a couple of hours at a time? Would that confuse my plants?

I just have plants in there right now ... no fish ...

(no fish *yet* I should add ... trying to get everything right before I put fish in there :-)

Good choice tommycat. Some people get over anxious and don't wait until things are ready. That usually ends up with dead fish or sick fish that you are desperately trying to keep alive, or both. This makes your experience a suck-fest. Does it have a light or some type of indicator to tell when it is heating mode or idle? If it has an indicator and it is still heating when the water temp. is above the set temp. then it is deffective. Return it for another. A 14 watt fluorescent bulb is deffinately enough to heat up a 7 gal tank as birdwidow said. Sounds like you have an eclipse or one of the hood/filter/light combos. Some of the newer ones have a cooling fan for the light, some don't. One way to keep the bulb from overheating the water is prop the light fixture up an inch over the top of the tank. Put a pin timer on the light along with one of those clip on the table fans (Wal-Mart or discount store about$6). This way when the light is on, so is the fan. Point the fan so it blows over the surface of the water, in that 1 inch (or even 2 inch) space. This really helps keep the lighting from heating up the water in the summer. 80F is fine for most tropicals though.