I'm preparing my 125g tank for 2 Oscars and 4 green terrors. I've read that in order to prevent rocks from being knocked over, you should consider gluing the rocks together... I like to create new environments (caves etc) every now and then, but I also want my fish to be safe. I'm using PFS as substrate... if I place the rocks soundly on each other are they fine? Any input/ advice would be appreciated!