aquarium heaters

Question
Hello. I have a 55 gallon tank with 2 small discus, 7 small angelfish, several tetras (different types), 4 albino cory cats and a bristlenose pleco. I plan on removing (giving them to friends) some of the angels in the future because I don't want to overload my tank.
I have added several real plants and driftwood. I am using sand instead of gravel. The man at the pet store suggested sand so I got sand instead of gravel. I like the way the sand looks because I want a natural looking aquarium.
I have a double hang on the back filter and check my water a lot. My water parameters are almost perfect and I do regular water changes. I also have a bubble wand in the back of the tank. I just recently added a power head because someone told me maybe my water wasn't circulating enough and then I had someone tell me my water was probably circulating enough for the heater to work properly. I just turn on the power head for a little while each day. I have never used a power head in the past. I don't like leaving it on all the time because now that I purchased it I don't really think I need it. I use it maybe 15-30 minutes every day.
I am having the hardest time keeping my aquarium heated to at least 80 degrees. I know the discus can handle it a little warmer. I want to get my water temp up for the discus. They were expensive and I don't want to lose them (or any of my fish). I have bought 3 different 200 watt heaters and none of them would heat the tank to more than 79 degrees. I have had this tank for awhile and just set it back up on Feb. 27, 2008. I have never had a problem keeping the tank at the desired temperature in the past. The tank has never been moved.
The heater is a submersible heater and it is placed in the middle of the tank near the bottom and last night I turned it ALL the way up and this morning it still only reads 77.9. The temp seems to fluctuate between 77 and 78 degrees. I have a digital thermometer with a probe and 2 stick on the side thermometers. They all read pretty much the same temp.
The first 2 heaters I purchased where Hagen Tronic 200 watt heaters. The man at the pet store said it was a good heater and the right size for my tank. When the first Hagen didn't work like I wanted it to I took it back to the store and exchanged it for another one of the same type because I thought it maybe had a defect but the second one did the same thing. I took it back and got my money back and then I purchased a Tetra Whisper 200 watt heater and it is doing the same thing. I was told that a 200 watt heater should be enough wattage for a 55 gallon tank. The Hagen heater said it was for up to a 65 gallon tank and the Whisper heater said it was for up to 60 gallon tanks.
None of the heaters would ever reach the temperature that they were (are) set at. Do you have any idea why the heaters won't even reach the temperature that I set them on? After using 3 different brand new heaters I don't think the heaters are the problem. I hope you can help. I am going crazy trying to figure out what to do. I really don't want to put another heater in the tank (unless I have to) because the heater is supposed to work up to a 60 gallon tank. Please help! I would just like to get the water temp up to AT LEAST 80 degrees. I just can't understand why the heaters won't even reach the temp that I set them on. I am so confused!
Thanks very much. Lynn

Answer
Hi Lynn,
I have to admit I've had trouble with heaters doing the exact same thing you just described. Its very frustrating. But I have found out that a lot of brands of heaters are just really weak or you really need two heaters to do the job right.

However, I have had the most greatest luck with a brand of heater called  "Visa-therm" heaters made by Marineland. There is another kind called the Visa Therm "Stealth" heater by the same brand and it is a shatterproof heater that has also proved to be very reliable.
They also shut off automatically when out of the water.

I would highly recommend these heaters above all others. I'm sure they would work much better for you and do their job well.

Best wishes and I hope this helps!
Karen~