Ziggy:

'xcuse me but..... needing a sump pump to get the nutes back into the reservoir (and another one to pump them to the buckets) is more complicated than one pump recirculating them.

The sump pumps you're talking about are good units, heavy-duty, relatively cheap for how much power you get. My $60 one [a Wayne] from Ace wil easily pump up to a 7' height above the pump, with a flow rate the same as my house water tap wide open. But........ they're hot! Left one in the res accidentally turned on overnight a couple times ---- could've cooked an egg in the reservoir soultion. :stoned:
So they couldn't be used for continuous duty, no way. Great for emptying reservoirs, great for watering soil grows (watering your pots with water/nutrients mixed in a reservoir).
Here's your float-switch links:

http://www.acehardware.com/product/i...Id=10509336467

and

http://www.acehardware.com/sm-wayne-...i-2151136.html