I can’t remember the last time I found a problem with Wirsbo PEX connections. Highly unusual.
Any thoughts?
Electrolysis and no di-electric fittings
The water heater has the dielectric nipples between the tank and the copper fitting.
As far as the fitting, the leading cause of failure in a brass fitting used with PEX is caused by dezincification. This causes the fitting to corrode and eventually create leaks.
I see no brass fittings. This is not the Zurn product.
From the looks, they soldered the PEX connector to a copper T. I would bet the leak is at the solder joint and not the PEX connection.
Possibility of a slight leak at the tank and Wirsbo coupling not seat right on the copper fitting.
Mostly prone to happen on the hot water side of things.
The deposit on top of the tank is directly below the encrusted PEX.
So I guess that eliminates the slight leak at the tank.
I thought so 
Below copied from:
Corrosionpedia explains Dezincification
Dezincification is the leaching of zinc from copper alloys in an aqueous solution. Dezincification is an example of dealloying in which one of the constituents of an alloy is preferentially removed by corrosion. The effect of dezincification corrosion is similar to graphitization in that one constituent of the alloy (zinc) is selectively removed, leaving the other (copper) behind.
This is from a home with a well and a non working water softener.
Water appeared to have a lot of lime and iron.
How did you came to that conclusion?
Rust stains in toilets. Unplugged water softener with no salt in brine tank and the large amount of calcification at the point of the leak and the deposit on top of the water heater and the prevalence of lime in wells in the area.
There you go, you answered your own question…
i wasn’t asking what caused it.
I was pointing out that PEX of the WIRSBO variety rarely has problems of this type so this one was rather unusual.