Here is the type of Check Valve I use this
I have it mounted horizontally and it works perfect........I needed one cause Having anti syphon holes on a dart with a single outlet would spray water all over the place and when I placed them lower in the water then my sump would overflow so I had to go with a check valve.
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500G Mixed Reef

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Electrician, Electronics Technician, I can help with any electrical questions you might have!!
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Kevin