I have two of the Koralia 3's on my freshwater now with pool filter sand it does take a little positioning to keep them from blowing it everywhere in the tank. I actually bought a set of Koralia 2's but found them to not circulate enough compared to the 3's.

So is your rock simply stacked or is it actually pinned together somehow? Guess I should check with Maelv to see how it was done...?