Apparently your water is pretty chemically active. Having dissimilar
metals (copper/iron) in contact is a bad idea in that situation (makes
a galvanic cell). Galvanized iron might buy you some time.

Teflon tape might help insulate the different metals, if you don't
overtighten it.

Seems you really want unions in the water lines, for quick disconnect.
With a valve just "above" each. Then repair/replacement of fittings on
the tank side can be done simply and quickly.

Chemical reactions are exponentially responsive to temp- lower is
better.

Maybe the fitting on the heater is damaged by torsional trauma
(over-tightening)?

HTH,
J