"But sadly the light developed a very annoying habit of shutting off on its own anytime there was a power draw somewhere else in the house"
Did that annoying habit show up about the same time the 1500 watt heater did? 1500 watt heater, 400 watt MH I think you're approaching the limit of safety on a 20 amp circuit. What else were you running on that circuit? Air
pump, basement
lights ect?
I'm a plumber not an electrician, but I"m pretty sure I'm right on this, please check with someone.