I was told to put Kitty Litter in my new speaker stands. Was he joking?

If not fill your stands with mercury. If you have any mercury left over mix it with equal parts vodka and lime juice and drink it while listening to the results of your labors.

Wouldn't be the best choice. There's a very good reason they stopped using those mercury filled glass thermometers decades ago...(mercury toxicity). Unless  of course there's some other kind of mercury you're talking about.

Put dried fish in your stands,are fish dampening ?

Gold dust can be used if you have some extra gold lying around. Make sure it's well sealed so it doesn't attract gold bugs.

Whether you use sand or kitty litter do not just pour it into the stand. Pour it into appropriately shaped thick plastic and seal it. Then you don't need to worry about leaks or moisture ingress.

The moisture absorption might be an issue.  I used lead weights (free from my local tire store) and added sand.

JUST AS IMPORTANT as the material is the level of fill.  I experimented with quarter, half and all the way filled.  Surprised at differences, settling on about 2/3.  Clearly, like everything else: try and experiment.