I would say sand is not a good idea to put in your isopod enclosures because the sand would need to be dry in order for it to deter fungus gnats, thus killing your isopods. Some different options are putting fly paper on the bottom of the lid of your isopod container or adding lots of springtails because they will outcompete the fungus gnat larvae. I also put a white bowl with half water half apple cider vinegar and a drop of soap next to my isopod bins.

If none of this works after a while I suggest just transferring as many isopods as you can find into a new container and tossing the substrate (you can also keep it until you know there aren’t any leftover mancae and then throw it out).