We are going to install a new liner in our 18 by 36 inground pool next Sunday - During the process of draining pool and repairing bad wall areas, we have disrupted the sand and it also appears that we will need to add more in the hopper area. What kind of sand should we use? We know from the liner installation instructions to use compactible sand and they say #50. No one seems to know what we are talking about at our home improvement stores - Any advice?