BTi larvicide -calling all microbiologists
https://www.thcfarmer.com/community/threads/bti-larvicide-calling-all-microbiologists.62595/
I like this thread, and apologize if this looks like a hijack.
I can add that my experience with excluding gnats from ANY access to media is a giant pita.
Sand is heavy, gnats just search out pot holes, and I just didn't think it was worth doing after throwing myself at it for some time, and viewing the gaps that open along the pot edge with any topping.
I'm currently attempting to dry the pots out between waterings. I'm told this will help. And Gnatrol.
edit: my gnats would laugh at hydroton as a barrier.