Never and I mean nver have any substrate in an aquarium breeding bettas. If youmust though because you have no other way to keep the plats down us a small container full of substrate that you can completely fill with plants other wise the eggs and young fry will become trapped under and will drown. Yes fish can drown no way to pass water through the gills, I had this problem when i started breeding bettas about 4 years ago and i havenever put substrate in the aquarium and now im doing great. I have a batch of opaque blue buetterflys on the way.