Ok, I'll try it. ?I don't think you have to replace the substate. But maybe float your val so it grows taller? Then plant it when leaves reach the top? Just a suggestion.
I'll be honest, when I first got my vals they were itty bitty things and just sat in my 21" deep tank. I gave them to a friend who had shallow tanks and he gave me a bunch back. The ones he gave back did really well in my newish setup tanks. I think it needs to get going before planting in deep tanks is what i'm saying.

Oh, ok. I got that.
