I supposed one could even use a big pond filter http://www.bestpondfilters.com/
if they had several large tanks. (Ka-Ching!)

"All" one needs, according to the write up, is a glass tube and a Bunsen burner - things we all just have around the house. ;) I had one of those, fashioned by an aquarist who also made neon lights. He knew how to heat and bend glass tubes like no-body's business. However it got broken pretty soon after I brought it home. Plastic tubing or even PVC might be better for many of us.

I don;t have pictures of it, but while visiting the Champaign Illinois club several years ago, we visited the home of a graduate student (in physics). He had about 7 large aquariums arrayed around two sides of the living room. They were connected by 2 inch (5cm) PVC pipes. The loaches roamed from tank to tank! He added CO2 at one point just after the filter. The aquariums were gorgeous, despite a little of that inevitable algae in odd places.

I searched Google for an image of an "automatic glass siphon." First hit was http://www.sciencefair-projects.org/technology-projects/automatic-siphon.html

A home brewing page has a simpler device, http://www.northernbrewer.com/pics/fullsize/auto-siphon.jpg
That is not too different from the hard aquarium airline I attached to a silicon airline (though short term there is no reason why one can't use regular airline. Discussion is here: http://ask.metafilter.com/8167/

Here is another version, set so the tank will only drain so far. That is important in case of a power outage.
http://www.aka.org/aka/modules/content/index.php?id=16

A big criticism of shops with a flow through system and sand filter (even though the sand filter may be very effective) is that diseases and pollution, if they happen, can get spread around. So if new fish (not previously quarantined elsewhere in a stand-alone tank) in tank 3 have the creeping crude, the fishes in tanks 4-100 may be exposed to the disease organisms before the sand filter rakes them out. :(

I know this from my miss-adventures. Make sure that all parts of your system are easily accessible. If something goes wrong or needs tweaking, it will be out of reach otherwise. ;)

Also make sure that it doesn't impede the flow of traffic around the house. I turned down the offer of a free sump for the living room tanks. I was rather interested in going on living. :0

By the way, there are different kinds of aquarium filtration. I'm guessing that you know this, but for others... here is a pretty good summary of many of the options. http://thegab.org/Articles/Filtration.html

You are probably talking about running a hose from the one tank to another and then from there to an intake on a power filter or a canister filter. The automatic siphon is attached to a hose and returned to the other aquarium. That then drains to the filter. Canisters work pretty well, partly because their ability to push water uphill (sometimes measured as "head") pretty well.

One can spend a lot of money with automated systems too. http://www.automatedaquariums.com/cont5sys.htm
You can even get systems run by your computer. If you Google multiple aquarium systems, there is a dizzying array of products which will be listed.

As miskairal, Angelee or guppygirl would have noted in the past, you are in danger of MFT syndrome. Multiple Fish Tank Syndrome doesn't seriously effect many aquarists, after a brief fever. But it is fun to dream.

And some of us get bitten. Like Malaria, it may go away. Recurrences are not uncommon though.

You may want to stop here. You got me daydreaming and Googling. The following seems to have been somehow typed. It was a pleasant morning. ;)

I though Googling multiple aquariums would produce more use-able images than it initially did. Googling Fish Rooms was more productive and you may want to do that. Some I liked were:
http://hometown.aol.com/missdionne96/index.html
http://www.aquarticles.com/articles/management/Shaw_FishRoom.html
http://www.bobhuels.com/fishroom.htm
http://www.aquarticles.com/articles/people/Norfolk_Jamaica_Hall.html
What the heck, see:
http://www.aquarticles.com/articles/people/index.html
(That is a very useful site)

Some specific Guppy and Killie rooms might include:
http://www.alfanita.com/fishroom_e.htm

http://www.chesapeakeguppyclub.org/Fred%20Hinkleman's%20Tanks.jpg

http://www.chesapeakeguppyclub.org/MR%20Fishroom%20photo%20south%20wall%201.jpg

http://www.geocities.com/michiganguppybreeders/Thefishroomofgaryandtimemousseau.html

http://www.geocities.com/michiganguppybreeders/GenoSoriceFihroom.html

http://www.geocities.com/michiganguppybreeders/EugenesFishroom.html

http://www.ifga.org/guppy_store/gs_shubel.htm

http://www.ifga.org/guppy_store/gs_white.htm

http://www.ifga.org/guppy_store/gs_schwab.htm

http://www.ifga.org/guppy_store/gs_roebuck.htm

ah… and look at this!
http://hometown.aol.com/missdionne96/index.html

Smaller tanks can be used too. This diagram is hard to view, but about fifty 5 or 7-gallon tanks were hung on a wall (you betcha they found the wall studs!) and run to one sump.
http://www.aka.org/chika/library/tankrack/tankrack.htm

Even there I wondered if disease organisms would move from tank to tank. The answer is yes, but if the water is kept pure enough (changed and filtered) the fishes' immune systems would not be compromised and illnesses shouldn't break out all over. If the aquarist gets busy, it is possible to mess up a lot of fish at a time.

That set-up was used for killifish. I also wondered if a pair spawning in tank #4 could influence neighboring, down stream tanks. Would sperm from a spawning pair in #4 drift into tank #5 and fertilize eggs, creating inconvenient crosses? Female killies have been known to pass a spot a male recently spawned in, have stopped and laid eggs. And they were fertile!

I would spawn pairs elsewhere and use those tanks for grow out. Or keep guppies in them. ;)

Sometimes a person will set up a wet-dry filter in a tank or in an acrylic box. That serves as a sump. A powerhead moves the water to one or a string of aquariums. http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=13648&more=1

Another version of this, on a smaller scale, involves a bunch of small fry containers held in a tray on top of a larger aquarium. There is an undergravel filter or a large sponge filter in the tank (which probably has no fish in it). The lack of fish in the tank, allows the system to respond to the needs of those in the trays. Also, jumpers may be more easily rescued and hopefully identified.

That filter system is attached to a small powerhead which pumps water up into a small reservoir. Gravity flow lines from the reservoir run to the individual fry "tanks". Both the plastic fry containers and that plastic reservoir have an overflow hole melted into each of them (probably done outside where ventilation is good). Then stainless steel or plastic mesh is cut into circles or squares which will more than cover the holes. The containers are placed on a flat, firm surface, with the holes on the down side facing the surface. A bit of a solvent designed for plastics (I forget which one) is placed on the inside of those containers around the hole. A little of the plastic will briefly melt. The screen is placed over the holes and pressed down so that it goes into the molten plastic. (Your finger or a tool or a wide dowel is used to gently press down it over the hole – not in that solvent, please! The edges should submerge in the solvent anyway.) As the solvent evaporates into the air (you can see why this is outside) the plastic firms up again and the screens should be firmly embedded.

http://www.aka.org/chika/library/flowfry/flowfry.htm

This is a really ugly photo, but it shows my cut open gallon jugs and some of those siphons I use on my tanks. The hard tubing goes in the jug; the flexible stuff is run to an aquarium. They are used to gradually introduce new water to a tank, which has had a bunch of water gravel-vacuumed out. Hopefully this will avoid any significant chemical stress on the fish. Just as importantly, the modest stream of water from the siphon doesn't stir the aquarium up or dig holes in the gravel.

http://pg.photos.yahoo.com/ph/unclescott@prodigy.net/detail?.dir=4b0fre2&.dnm=e7eere2.jpg&.s rc=ph

This is a shot, on water changing day, before I began rearranging things - pulling out the cinder blocks and 2x4s on one side and replacing them with Gorilla Racks. (14 square feet of tank space were replaced with a usable 30 square feet of tank space and another couple of inches on the sides of the tanks.) Several of the cut-off gallon jugs are in position and have been siphoning water into tanks. On on the battery jar of greenwater and Endler's fry (yep, a little bitsy thing, but look what they are going for on E-bay) is just sitting there. The siphon lines are about impossible to see, perhaps mostly to the other side. The floor is much cleaner, drier and a little less cluttered these days - at least for the time being. ;)

http://pg.photos.yahoo.com/ph/unclescott@prodigy.net/detail?.dir=4b0fre2&.dnm=8e15re2.jpg&.s rc=ph

Multiple tanks like those can have air delivered by a system of pipes, hanging from the ceiling or attached to the walls. The wider the diameter of the pipes, the better. The 1/2 inch pipes I have there are not as efficient as 2" ones would be, but I was able to get them inexpensively from a shop that was (unfortunately) closing. They were already drilled, tapped and had valves in most of the holes. That was pretty important.

That first hideous image shows a length of piping   extending in a T with piping hanging from the ceiling. That is held by monofilament fishing line, hung on decorative hooks screwed into the ceiling beams or to the frame which holds the dropped ceiling. Not long afterwards, I installed a small linear piston pump, probably the most efficient type (cheapest to run) air supply system for a bunch of tanks. That replaced my Sweetwater blower. Within two or three years, the purchase price will have paid for itself because it uses relatively little electricity. In time, hopefully this fall, that system will be expanded to a motley collection of 60 or so aquariums.

This was the original source of my linear piston pump.
http://www.jehmco.com/html/central_air_pumps.html
I bought one of the smaller ones, the Model #LPH45. It has been used less than a year and it went at auction for 1/3 the original price. I felt badly that it didn't go for more, because the guy selling it was a teacher who had used it with an school aquarium club until school policies, including flip-flopping him from room to room, made the club very hard to keep going. On the other hand, nobody else there would keep bidding and it is awkward or downright weird bidding one's self up another 25 or 50 bucks.

If you Google professional aquariums (those could be the Shedd-SeaWorld-Tennessee-Atlanta-National Aquarium sort) and how they work, you may get other ideas. Sites on fish shops and building a fish shop may also offer insights, even though what you and I do is on a much more modest scale.