An aquarium heater may not always do the trick if it gets cold enough, and so I also have on hand the following low-wattage heat solutions: waterbed heater pad (small footprint, very adjustable, can heat a much larger volume of water that's set on top), and my infamous Electric Blanket Sandwich, which is an electric blanket folded up and inserted into contractor's bags, pillowcase-style, sealed up with tape, and this is better when you have a large surface area that needs to be kept warm.
Since my cloner goes into my veg-table, and my veg-table has the EBS (Electric Blanket Sandwich), I use that set-up to help keep temps in range, and the aquarium heater keeps my mix water warmed up a bit. The waterbed heater does the best job of keeping the large volumes warm when the aquarium heater can't keep up.
The waterbed heater is about $55 or so (I got the type for softside beds), and the electric blanket can be free if you've already got one, otherwise about $30 at Walmart.