There's two ways to deal with this... the expensive unreliable way is to buy a PVC one way check valve that will almost certainly fail eventually or you ca simply raise your return line to a level just below the tank water level so that when only a small amount is siphoned out of the tank and into the sump air will fill the line and break the siphon.
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135G Mixed Reef. Bullet 2, 25 gal refugium, 2 X250W MH + 4X 96W PC\'s, DIY Calcium Reactor, Coralife 1/6 HP Chiller, Phosban, Tunze, 2 closed loops & SQWD\'s, Seios, Coralife 4 stage RO/DI & a bunch of other expensive gadgets... I may never retire, but I'm gonnahavahelluvanaquarium!
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