Thermocouple set to J is normal and seems thermocouple is working fine. Do you know where the thermocouple sheath (end that senses temperature) is in the pod? is it near or far from the heater?

When you say heater is in back, is it an inline heater (heating element is in the plumbing and only heats while pump is on)? If not, is the heater on the floor in the back or in the wall?

If the heater is not inline and is in back, it could be a faulty design to begin with. If only the back portion of the pod is heating, it will create a heat disparity between the back and front which would would result in poor float environment.

If the heater is an inline heater, than most likely the calibration is off and simply needs to be adjusted with the controller. Could you upload a picture of the controller so that I can see it.

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Salt density. Yes, 1200 lbs of epsom salt is a lot. It is 1/3 more than a Samadhi at maximum specific gravity, which I don't even like to float in. Do you have a hydrometer (specific gravity meter) to test? at 1.3 specific gravity, the solution is maximum saturation at 30% epsom salt. This will create a "stiff" solution which tends to produce more neck pain as the head falls further back. Also at maximum saturation, you run the risk of recrystalization which sounds like you are experiencing. Never let salt crystals go through the pump as they scratch the impeller and diminish efficiency (and makes the pump noisey). Usually a hair trap before the pump with a good filter medium it it (like the blue aquarium filter medium rolled up and inserted) will keep that from happening.

I think 1.27 is optimum and reduces the chance of recrystalization. Whatever the solution density, make sure the pumps are run for, at least, 15 minutes twice a day to keep stalagmites from forming.