Usually people need res chillers so you pretty much have to be at or below 60 to need a heater. Here is a bit of wisdom from another grower:

The oxygen content of a fully aerated solution at 10C (50 F) is about 13ppm, but as the solution warms up to 20 C (68 F) the ability of the liquid to 'hold' oxygen drops to 9 - 10ppm, by the time the solution has reached 30 C (86 F), then it's only 7ppm.

Plant roots want oxygen.............
For cannabis you should keep the temp around 60-65, so unless your below that I would say that low temps are not your problem. Even so I have never heard of your type of problem so I would mix a bit of your nutes in a warm glass of water and see if the same thing happens. It could just be your nutes.