I have a question about a Hot Springs Sovereign II (2013) hot tub. Do all 4 filters work ( i mean , are they filtering water) when only the circulation pump is on ? Or does only the 2nd filter in (with the grey top cap) only filter the water ? I know all 4 filters are working, when the jet pumps are on, but am curious to how they work, when only the circulation pump is on and ozone.