Thanks Sam, those are 'river rocks'...about 1/2 to 3/4 of an inch.
Yeah, those lava rocks are tough to work in...

You might think about setting up some new tubs, filling them about 1/2 ways with the lava you have...then top off 'all the tubs' with some river rocks.
It's win-win for you Sam.... :P ...you get to expand your system and get rid of (or hide) the lava.
We dso that at Sahibs...the lava keeps the weight down.