It is likely that either the water level in the reservoir is too low allowing air to vortex into the suction, or the suction screen on the pump maybe clogged/restricted creating low pressure in the pump which causes dissolved air in the water to come out of solution. Reducing the pump flow rate may help if you cannot find another solution.
Sometimes freshly made water can have high levels of dissolved CO2 that will degas and then gradually go away, but I have only seen this on my freshwater tanks. Normally the process of making salt water and storing it will allow the water to degas.