i am well into this project and so far its going ok , i'v built a rock retaining wall around the entire feature with a small upper holding pond above the waterfall that empties into the pond that i have dug down to bedrock with a number of shelf's and with the retaining wall i have just under 9ft of depth. How ever only after i have laid in the liner and started adding 3'' river rock i now realize my shelf's are not deep enough also my choice in rock wont allow me to build up to the shelf's without filling the bottom up and loosing ruffly 3.5ft of depth. what i am wondering is what kind of mortar can i use that's fish friendly to build up larger rock shelf's throughout the edges so i don't have plain uncovered liner. i will of course be using scrap pieces of liner under anything i'm building to prevent leeks ??