How Can I Heat My Reservoir Solution to Keep it in the Correct Range?

Submersible Aquarium Heaters

Use a submersible aquarium heater in the reservoir. Use only the common type which have a thermostat built in, and only the type which are fully submersible. The glass body of the heater must never come out of the solution or the glass will crack and the heater will short out & burn out.

If you have the type of e&f system which uses most of the res capacity to accomplish a flood, you must do something to be sure the heater never goes dry during the floods. The most common and best way around the problem is to place the heater in a glass, pyrex, or corning ware dish inside of the reservoir so that even if the res drains completely the heater will stay underwater.

If you can't locate a big enough lasagnia or cassarole dish, another method is to have the heater on a timer to shut off about thirty minutes before the flood, so the heater cools down well before it goes dry, turning it on again after the scheduled floods have drained back into the reservoir. If you must use the timer method, you'll run the risk of the timers getting out of sync unless you use the digital types with battery backup.

What is the right temp?
I looked at the submersible aquarium heaters and they maintain about a 78degree (F) temp.
Is that too high?
I have read that 70-75 is optimal.
Will too much heat?:
A: Prevent growth and reduce harvest?
B: Cook the roots?
C: do any long term damage to a mother?

If they are fine they are a great deal PetCo and walmart had them in the $10-20 range depending on gallon size of tank.

It's best to use two smaller ones in case one fails. The most common failure mode is for them to stick on, thus a 100 watt heater will rapidly overheat a small sump. While two smaller ones will do the same job, except if one sticks on res temp won't rise so fast, or get so hot overall. Besides, two smaller ones work better in the "lasagna pan" trick above. Figure 3-5 watts/gal for each ten degree temp rise needed as a ballpark figure. Google aquarium heater size calculator if you wanna get anal.