I had to do the same thing. I pulled the rock and left the sand in. Just a bit of water barely covering the sand. I could not believe how heavy it was!!! Good thing I had a buddy close by, no way I would let my wife even try to help with that move.
I pretty much gave up on the idea. I moved some of the rubble into the main tank and put the old sump back in.
I'm returning the 20L today, and ordering another hydor koralia powerhead. I just got one the other day, and I love it! Works way better than the standard style powerheads, and looks cleaner, too
I have to agree the Koralias are nice. I also like the maxi jet 1200s for stuff like mixing salt in garbage cans and other utility chores.
Good luck with everything.