I'm by no means an expert-but i noticed that even in tent my 5 gallon buckets would run below 68 sometimes (i've since added 100w fish tank heaters to my buckets since they can ~provide up to 10 degrees above relative temp... so they say). I've read that you would want to keep it higher than 68 to prevent slow growth but up-to/below 72 to prevent the bacteria from infesting (well... ones you didn't intend on using). I know they are kicking on because my temps are steady in that bucket.
I would assume the larger reservoir needs it more than this small bottle-that is why i'm not sure it will need it. The more water-the longer it takes to heat, reheat, and maintain temps.