Solar controller common positive, floating neg?

[Foeboat]

I have not seen this discussed before but wonder if I am thinking this through correctly ...

Last season I upgraded to a new MPPT solar charge controller (150 W panel) and, upon haulout, I was stunned to see that after a short time (6 months) my anode was completely wasted. In the past 10 years, I've never seen this on this boat so I suspect that the solar installation is at least part of the problem. Then I discover that the ebay MPPT controller that I newly installed has common positive whereas, like most boats, the negative side of the battery is ground. Surely this is a problem as it leaves the negative side of the controller output to float relative while the negative side of the rest of the boat is connected to the boat's ground (engine block, prop shaft, etc) and ultimately to the zinc anode. Am I looking at this right? What is the correct fix? (I know that negative common controllers are available but splashing out for a replacement seems rash!)
Thanks!

[john61ct]

I would only use a positive-common device if I had a very compelling reason to.

Like adding it to an electrical system that was already set up that way.

Sell it on eBay, cheap lesson learned.