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A TG nutrient solution tub heater should be fully submersible, shatterproof (no glass), rated in the 200 to 300 watt range and an accurate temperature control that includes the TG's optimal nutrient solution temperature range of 65 to 75 degrees. Here is a link to the heater I currently recommend:
Aqueon Pro Heater
In all cases, you should maintain the TG nutrient solution temperature in the 65 to 75 degrees F. range (favoring the lower end of that range when possible). Your growing season is over once you can no longer stay within that temperature range.