you can use a rheostat for a temporary fix till you save up for a thermostat. Flexwatt heats up fast and HOT. i tested my new ones today after wiring them and they hit 132oF. rheostat on them will be tricky but not impossible as long as room temps don't fluctuate a lot.
I have 2 tanks with flexwatt on rheostats, as long as you have stable temps in the house it will be fine, mine will fluctuate between 92-96, which is acceptable
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