Just going to throw this out there for what it is worth. While I do not have the tank you are talking about I do have a overflow in it that I had a gyre xf250 mounted. The external magnet was inside the overflow since April of this year until I recently removed it this past weekend. No visible signs of rust and it looked more or less in great condition. However...
I have been chasing a few minor issues with my rose bubble tips not showing their earlier color and some hammers also looking paler and not expanding. New tank, 210 gallon, completed fishless cycle in March. Turned on lights and skimmer on April 1st. Moved my 40 breeder into it on the 15th of April and it has been running ever since. I know it still is maturing - I used 150 lbs of dry Pukani - so figured I was maybe chasing high nitrates. In any case on a haunch to check my numbers I sent off an ATI water sample. It came back with lower salinity measurement, tin, and high zinc.
Salinity was a easy fix and it wasn't that far off. Tin and Zinc was another story because I don't use any metal in the tank. If anything it would be magnets and my heater. Heater is new (not that it matters) has are the magnets on the gyres, algae clip, and tunze glass cleaner magnet. I was reading around and one of the general themes was tin and zinc could be from a magnet if nothing else metal is in the tank. Light goes off, I have the algae clip, glass cleaner, and one of the gyres in the tank. This past weekend I removed everything, performed a water change, will run another two changes this week (Tuesday and Thursday), then will send off another ATI water sample.
It isn't in my RI/RO, no metal clamps, barbs, nothing. Only thing it could be is one of the magnets or so "I" think. At least that is what was common from reading over the weekend. I reached out to Maxspect and they said it is fine in the water. I'm not going to be one telling them they are wrong especially when I didn't see any damage. However, I can't explain why my tin and zinc was high other than another possibility is hand lotion with high zinc. I usually wear gloves but maybe it is leaching through...
Anyway something to consider. I've since removed mine and will place it either under the overflow or position it differently on that side wall.