Hi swanzyusmc, welcome to the forum...

Your grow tubs 'may' stand up to lava rock, but I don't think they can handle the weight of river rock. They will bulge and may even split. Not something you want to come home to...
If they become an issue, Home depot has mixing tubs in the section where they sell cement. They have 2 sizes of tubs, the smaller one may fit on your stand.

I see your drains are tied to a common header. If your trying to do bell siphons, you may find out.... if one tub is draining, the 2nd may not be able to start, till the 1st is finished.
If that happens, and your not happy with it....make each tub, have it's own drain. Just remove the 'tee' and replace it with a 90. Both tubs need a horizontal pipe about a foot long, and a second 90 pointing towards the fish tank.
The 2 90's, create a little back pressure and helps the siphon to start easier.

Good luck and keep us posted..