Ken, when a heater says it's factory-set at 78 degrees, does that you can't increase or decrease the temp yourself?
If it's a dail type you can increase or decrease the temp yourself.. 78 is just where they tune it to when built.
I think GoldwingDon has you covered there.
There are some heaters that you can buy that does not have the dial and I think it states on the package as to what temperature it maintains. You put it in the tank, wait 20 minutes, plug it in and it heats to 1 certain degree and that's it. It won't go any higher or lower. There is no temperature range.
I believe these are mainly used in very small set ups.
I've moved your question to a thread of it's own as it was posted in a fairly old one.