I'd deal with the problem and not just let it drip. You'll end up with a big salt creep problem. Don't try to clog the hole with sand. Won't work and will make the real repair even worse.
This sounds like a reef-ready tank with built-in overflows and both drain and return line routed throught the bottom pane...true? If so, I'd turn off the return and allow that overflow to drain completely down. Remove the bulkhead and figure out what went wrong.
Was this a new tank or one that been set up before? If the latter, buy new gaskets (or entire bulkhead sets) for all four. The bulkhead gasket is on the flange (non-nut) side of the bulkheads, true? Depending upon the manufacturer, the gasket may have a couple of small concentric ridges on one side...if so that side should be to the glass. Tighten only hand tight or just a hair more by wrench.
