When I had the tank down fixing a leak I attached a small piece of acrylic to the bottom maybe 1/2" cubed using weld on. I only bonded just a small piece of corner so if I ever want to remove I can just break off with some pliers. I then drilled a hole in the block and used a small zip tie to attach the plants to them. They are not going anywhere now. I can just cut tie and replace when I need to remove and clean them.
Thanks, that means a lot coming from the resident marble guru. They were definitely money well spent. The aparicio has been caught following the bigger female around like he is sniffing her. Who knows, could be interesting in the near future