With this particular tank I have always struggled with acros. I think the Tin was in there from the beginning and didn't suddenly appear.Unfortunately I can't rule anything out really, but the tin on float glass is a layer. It doesn't leach, so I wouldn't expect it to suddenly appear. If you'd not done water changes since starting up, then it's possible, but I think unlikely given water changes.
You are right in that corrosion would result in more than one metal. Since tin is a common plating metal, I would expect to see Nickel, Chromium or Copper showing too.
For a new tank, wash everything down (wiping it) with citric acid solution. Do this with pipework and anything plastic. PVC plasticisers and stabilisers are probably the biggest source of tin contamination, which is why I asked if you'd added any new equipment.