How do I raise res temps?

I use an RO fiter since my tap water is horrible and my water temps when filled are about 61 F for 50gal res which I know is too cold. the plants are fine it's not shocking them but I have to raise it. any simple ways without killing my electric bill?

use an aquarium heater.

Wouldn't the water be room temperature after acclimating? Is your room 61d?

Put a few water pumps in them. Will keep temps up and will keep your rez well mixed

If the roots are fine and plants growing why raise temperatures? 9 degrees higher and you're going to invite problems. People buy chillers to keep temperatures well under 70.

I use a aquarium heater in my rdwc. Keeps water at 68 when ambient temps are lower than that. That said, I run sterile.