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Not something we'd recommend. The original design of the water tanks was that you could get in there and wire brush it out when needed and apply another coat of water tank paint. That way you can visually check the condition of the water. I think Brian possibly had Tony at the yard to do it in latter years, at the same time as the 2 yearly hull blacking.
Loose tanks lead to condensation between the surfaces and more maintenance in the end. We used to fill through the lid (not sure if there was a separate filler) until the water overflowed into the cockpit.
We tended to fill up quite regularly to avoid tanks full of less tasty water (or test the water first) but of course you'll be aware of all that.