I used kitty litter as substrate for most of my water plants. I only have 1
pot of Water Parsley with garden dirty, when I couldn't find left over.

Elaine T wrote:

That's simple enough. I'll use up the extra Fluourite since it would
just sit around anyway and then go with something cheaper once its gone.
Once I get marginals and a lily, my understanding is that they want
real dirt anyway with gravel on top like Roop said.

Unfortunately, I just fertilized everything last week with timed release
granules so dirt from my yard is probably not the best choice. I'll
check my pond suppliers to see what they have or grab some plain potting
soil.