Java Fern "black spot" and translucent

>From: amc2 at ix_netcom.com (MERRILL COHEN )

>Let me give you this "old-timers" remedy to your problem -- get an
>ordinary clam shell (or some oyster shell) and put in in your aquarium
>or in the filter.  Watch the change in one month!  You can always
>remove it if it doesn't work, but I bet you will leave it in your
>aquarium forever!  Better than adding chemicals and upsetting
>everything in your aquarium.  The other plants will love it also.
Merrill, what is the reason behind this advice?  I've never heard of
anything like it before.