"Ray @ First Rays Orchids" writes:
If the medium you used is very light or substantially spherical, it may be
tough to get sufficiently stable. Look around for a brand that's more
irregular in shape, and it will lock-in faster.

It is. No other brands are easily available. Do you think mixing with
marble chips would be an option? They don't retain water, but they are
heavy and irregularily shaped.

I guess I could also use a lump hammer to break the spheres up a bit.

Geir