Yeah it's really a question of the state of the heater components and not the plastic. IMO there's no reason that hardened plastic should now leak nutrients.
It was melted and hardened before, no difference. If you really were worried about lose plastic pieces, scrubbing it with a greenie would work.
To test if it works just put it in a bucket with a thermostat as well.
additionally people are mentioning worry about stray voltage, so you'd have to beware of that/check for it.
Otherwise it would be totally fine,
However seeing as a good heater is only $50.
And if you are a dedicated reefer you've got a couple grand sitting in that tank...
Why risk it for $50?
It's like driving 6,000 miles on the same oil wondering if you should change it yet.
Probably. You could probably make it to 10,000. but the OEM's recs are 5,000.
