I believe C41 stabilisers contain an amount of Formalin which is not in itself soapy but the problem you are having is with the very soft water. Almost anything will make bubbles!
The amount of froth created when you have a shower will show how soft it really is.
You could try diluting the stabilser with bottled water which will in all probability come from an area where the water is not so soft. Diluting the stabiliser further than the recommended will probably just make it less effective.
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