I have about 2+ inches of fluorite substrate in my 6 gal that contains
one crowntail betta and two African Dwarf Frogs. I haven't had any
health problems since the tank was established in October 2004. Frogs
are generally pretty sensitive too. I've been slowly converting all my
tanks to sandy bottoms but that because 1. Plants stay put better and
2. I prefer the aesthetic.

Very interesting theory though. Let us know if you hear any more about
it.