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What chemicals are you using now in the tanks? Can you attach a photo or two of the area you are referring to so I can take a look at it?
I was hoping for some photos of where you thinj the leak was coming from.
From what you told me it does sound like the tank may have a leak and you are going to have to get up close to it to see where. That can be a chore. It isn't likely to be leaking from the top or anything having to do with the valve.
Kind of what I told you here.......you have to get to the tank and inspect it. There is no "one" place they all leak. In my shop we have to do it exactly the same way. Plan on about $300 or so for a tank plus some fittings and a messy job. We do these all the time. Good luck with your project!