Get yourself registered on ReefCentral.com lots of good resources on there for configuring / information on saltwater automation.
Remember that once you've disabled the siphon by blowing air, your return pump better not be pumping any more water from sump into main tank until the siphon is re-primed or a couple of things might happen...
- Sump is going to run dry on you and you'll need a new sump pump
- You might overflow your display tank
Talking about RODI etc - If there's a local reef-club, get yourself logged in, see if anyone is getting out of the hobby. First rule of reefing - BUY USED if you can
We can trade tank design scenario's etc on ReefCentral (I'm "BigFloppy" over there).
Lets keep this thread on track with your arduino buildout. The people in this forum can and will help guide you through the programming mire.