If the pool stops draining at the tile, then your primary, but not necessarily your only, problem is at the tile line.

If you really want to be specific, use the poly-quat and turn the pump off. Then, if it stops draining at some reasonable level -- don't let the whole pool drain -- you can turn the pump back on (if you have a main drain) and see if it drains further. If it does, you have BOTH a gravity leak, probably in the shell, the tile or the piping from the skimmers and/or main drain AND a pressure leak in the piping coming FROM the pump.

In many cases, even rather large leaks will not show up above ground: the water tends to flow down, not up.

Good luck!