Thanks for the suggestion. Will try itYou can't let riccia grow into "a nice large ball" as it will break free if the lower portion does not get enough light and dies off. I tried riccia rocks for a bit but slacked on trimming and they all broke loose. Lava rock or slate are two easy to obtain cheap rocks that are inert. I'd recommend getting a shower sponge/scrunchie and using that mesh to hold down the riccia.
What kind of rock do you use ?I tied mine with sewing thread nice n tight to rocks, everytime it's getting ready to outgrow the stone, I trim and tie it onto a new one.
The shower caps work good too, but I like regular old thread more and it's easier to work with IMO. I do the same for subwassertang.