I use a pond pump (Pondmaster brand somewhere in the 900 to 1200 GPH range...can't remember right now) attached to a garden hose. The pond pump came with a plastic guard that fits over the intake to prevent large-to-fairly-small stuff (like fish and wads of java moss) from being sucked in (small fry would still get sucked in). I have a convenient showerhead that I can attach the other end (with an adapter and quick connect hose fitting) to if I want tap water, otherwise, I have another pond pump that goes on the other end to pump water from my DI storage back to the tank. Another thing to consider is that I've *heard* stories about antimicrobial compounds being added to some garden hoses. So just because I'm a spaz, I got hoses rated for potable water. You can get them in any Big Box Hardware store in the garden center, labeled for RV use and they're usually white in color.
I'd also recommend a hose shutoff valve between your pump and the hose for when you need to turn off the flow quick and can't get to the sink or plug fast enough. It's also good on the refill for going between tanks quickly without having to fold and pinch the hose to shut off the flow.
I'm lazy so I leave my big pond pump attached to the end of the hose and don't even take it off for the refill. I used to use hose quick-connect fittings so that I could snap it on and off but I've yet to resolve two issues with them: 1. they eventually get leaky when they're flexed too much 2. they narrow the bore of the hose, so you really only get as much flow as can fit through the narrowed quick connect fittings. This is also a problem with the shutoff valve, but I found one that's a "wide bore" that's doing pretty well:
http://www.dramm.com/html/main.isx?sitesec=1.1.2.0 third item down, the Dramm Heavy Duty Plastic Shut-Off Valve. I sometimes find them in the nicer garden centers around. The Big Box Store hose shutoff valves all fail after about a year (trust me, I've gone through about a dozen of them between the fishroom and the garden outside...I've tried all of the Big Box Store varieties)....they stop opening and closing (I think a groove gets worn in the nylon ball inside). So go that route for a quick, cheap short term solution but keep an eye out for a nicer one down the road.
So here's how I do my waterchanges:
To drain tanks: put the plastic guard on the pump which is already attached to the hose. Drop pump in tank, run the hose to wherever the waste is going. Plug pump in. Make sure shutoff valve is open. When done, move to next tank or unplug when finished. For top tanks, I'll turn the pump on to start the flow, then turn the pump off and let gravity do the rest (using the shutoff valve to move between tanks without losing the siphon).
To refill: remove the plastic guard if necessary (sometimes it gets in the way) or leave it on....doesn't matter. Leave the pump attached to the hose but unplugged and shutoff valve open. Take the other end of the hose and plug it into the water source (quick connect fitting to the shower or pump for the DI reservoir). Refill as needed, using the shutoff valve to stop the water flow between tanks.
I used to use two hoses....one for fill and the other for drain, but I found that it just didn't fit into the flow of how I do things. So, a few other things I had to do to make it work:
I actually need two potable water hoses to reach the back door if I want to use the waste water on the outside plants. No biggie, just hook them together, but know that the longer the hose, the more pump head height rating you'll need to push the water through all of that hose.
To fit the pump for my DI reservoir on I had to get an adapter at the Big Box Hardware store to make the hose ends match up (I think I had to get a M to M adapter).
I like pond pumps that are threaded for a garden hose....lots of adapters at the Big Box Hardware stores for garden hose-to-whatever-you-want so it opens up a lot of options, IMO.
I also use a multi-output hose shutoff valve on the showerhead that I use for refilling my tanks, that way I can open and shut the one I'm using for refill while leaving the one for my DI refill open.