What`s the lowest voltage pump you know of?

I am not sure exactly what you are looking for but voltage is not something to compare pumps with. If you are talking about efficiency then you would want to look at the wattage. Theoretically you could get a pump that runs on 480 volts that could have lower amperage than your ones you run now but that doesn't mean it will use less power (wattage)

Then you could get into DC pumps...

 

Tunze pumps are 12 volt and consume very little power.

 

Are you looking for a flow pump for inside the tank or a pump to pump water through tubing i.e. a return pump? You question is very vague as to what you are trying to accomplish. You can find pumps that use just a couple of watts and would cost $.10 a month to run but move very little water.
FYI 5 amps is 625 watts and that is a BIG pump! A 5 amp pump running around the clock will cost you about $70 a month to run in electricity alone.

 

MP10 uses 8-18 watts and is also 12v.

--AJ

 

If you look in the Foster and Smith catalogs (if you get them) they have a list of all the pumps and what they're wattage is along with how much flow 0ft/3ft/6ft etc makes it really ea sy to compare pumps and figure out whats the best deal.