Either position the heater in a separate sump or chamber, behind a partition in the tank, or failing this provide a shield around the heater that allows for good water flow but prevents anemones attaching to it e.g. a perforated PVC pipe. The biggest problem with the tank mentioned in this letter is the lighting system. Like other reef inhabitants, many of the sea anemones sold to hobbyists contain symbiotic algae called zooxanthellae. This means these animals require light, and lots of it. The two twenty watt fluorescent lamps on this tank were far, far from adequate and this was most likely the reason for their death.
Anemones require bright light in order to survive.
At least two VHO lamps should have been used on this system, at the very least. Provided one can adequately cool the aquarium, placing it close to a window where it can receive several hours a day of sunlight is also very beneficial to sea anemones. In addition to the above there are a number of things that should also be done in order to increase your success rate. Although I mentioned that anemones have zooxanthellae they will feed when offered food. Sea anemones also seem to do best when trace element solutions are added, especially those that contain iron and zinc.