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I just did a purge with ahhsome but had nothing but major foam. No scum or buildup of gunk anywhere. The tub was new in May and I did the first change in July without ahhsome. Water has always been clear and I was expecting at least some gunk with the ahhsome but I was very disappointed.
Congrats, keeping small tubs clean and clear can be a challenge. I did the flush and fill only twice in 10 years. I'm guessing a normal refill will be just fine to get you through to the spring thaw.