Gigas grow very rapidly when given correct conditions. If you put it on the rock work, it needs to be very stable. As they grow, they use gravity to stay put, not the byssus gland.
Is it too much for a 120 gallon?
My 225 gal. is dominated by a 6 yr old gigas that started out as a 3 incher. It is now about 18" L x 11" W x 10" H. I had to redesign around his growth. I'll let you decide if 120 is big enough! Besides, if it does outgrow the tank, you have an excuse to buy that "really big tank" you always wanted!
