RO Reverse Osmosis Inline 450 Flow Restrictor Reverse Osmosis Quick Connect InLine 300 Flow Restrictor. This flow restrictor is to be installed in the drain tubing. The restrictor has 1/4 inch Quick Connect fittings. A flow restrictor will create back pressure on the membrane and allow the membrane to produce purified water to be delivered to the pond, aquarium or water tank. Flow restrictors are sized to match the flow with the size of the membrane. In selecting a flow restrictor, be certain you do not "under size" the restrictor as you will prematurely foul the membrane. If you "over size" the restrictor, you will be wasting water but not causing damage to the membrane. We recommend you replace your flow restrictor the same time you replace your membrane, as these can become clogged over a 12-18month period. Most membranes require replacement after 12months, so it makes sense to replace the flow restrictor at the same time.