It doesn't have to have a separate breaker box, just it's own breaker or two, like one for the basement equipment room and one for the upstairs tank circuit would be nice. but you know, with the housing market slow down, i'm sure the builder would be happy to add a separate small 2 or 4 breaker box and run and mount the basement wiring and GFI outlets right now. or at the worste add a few few bucks to the bill.
If you do the upstairs later you will have to locate the studs, cut the drywall, pull wire from the new breaker box, and install the GFI outlets and connect yourself. or hire an electrician at greater expense. so if you can figure out where to putthe tank, its much easier to get the wiring in now. and have it county inspected by the builder. you know I would consider 4 outlet wall boxes. you never have enough outlets to plug into. so like a double box on either side of the tank. 8 GFI protected outlets.
just keep in mind the load capacity of the line when plugging stuff in. total WATTS divided by 120 VOLTS = AMPS. both outlet boxes will probably be on the same dedicated tank circuit to one basement breaker. normal house circuits are 15 amp capacity, or about 1800 watts max. I would run a little less on them, like 1500 for safety. Estimate how many watts of lighting and pumps and things you might run on the tank. 15 amp should be plenty for let's say a pair of 400 w halides and lots of little powerheads and fans. the heavier load pumps and skimmer and possible chiller(i just use fans) would be on the basement circuit.
remember, to pump water from a basement sump to the top of the tank on the floor above takes a lot more power and watts.
so I would ask him to do the basement box and outlets now. that way the connection to the main house box will be done, and you will only need to connect one circuit later.