I would make a heater part your basic equipment to lessen the temp swings. I dont know about the HA, if the cheato is growing fast there may not be any nutrients left for the HA. Of coarse you would have to dose nutrients. I've seen several people with greenhouse who grow chaeto, and they harvest it by the 5 gallon bucket full. Also unless your tap is chlorinated you could just top off with tapwater, and perhaps lessen the need for nutrient dosing.