Wijgerde saw problems with Stylophora pistillata at 100 micrograms per liter zinc. Triton's test reported zinc at ~175% of true value (according to Ross in reefs.com), so even with ICP testing now available it seems to be difficult to know what the level of zinc actually is but it seems certain that low and high concentrations can cause issues with zooxanthellae. So, to answer your question about zinc dosing and algal filtration... I don't know.