My 300 gallon Drop Off Reef Tank Build

[Gripenfelter]

You only need 3.

At 5700 GPH my main siphon is 95% open. If I made it 100% open it would suck the overflow dry.

The way you set it is to set the main siphon so it is slightly slower than the flow of water entering the overflow.

Then you set the secondary open so it will suck up just enough to keep the water level from rising. If the water level rises to the point where it reaches the airline, the secondary also goes full siphon.

If for some reason the first two are clogged the emergency, which is never used, will go full siphon.

So really there are 2 fail safes. Anymore drains than this are overkill.