Hoa: I bought an all plastic valve at my local aquarium store for $1.50 or so, works fine and no sign of deterioration. And it is checked every time it is used (twice daily, don't ask, it's a long story). And it works every time. No visible deterioration in six months, nor is there any reason a weak acid like carbonic should attack a plastic valve. Dave Dave Gomberg, Experimenta San Francisco CA USA gomberg at wcf_com