"Fishless" Cycling. In an article first posted on the Internet in January 1999, Dr Chris Cow refined and popularized the technique of using a solution of ammonia (plain non-sudsing 10% ammonium hydroxide, if you can get it) to prime the bacterial "nitrifying" processes that metabolize ammonia to comparatively harmless nitrate in aquariums. You know all about this already, but you can brush up by re-reading his article "Fishless cycle", now archived at SimplyDiscus. A very clearly-written article by James Koga, "Using household ammonia for humane cycling of a tank", recently updated, adds some refinements to the technique that you might go for. Koga offers many links to the fishless cycling process.
There are other sites now that explain "fishless" cycling, often with additional touches that might help you: find the current ones by googling "fishless+cycling". I'll be figuring that you have grasped the basic concept of adding ammonia to jump-start the nitrification "cycle" that "cycles" a new tank, as I offer a few further points.
First of all, many fishkeepers seem breathlessly impatient about the length of time they're willing to allot this necessary, natural process. Don't rush it.
In the first few years while the techniques of "fishless cycling" were becoming established in this newly-developing tradition born on the Internet, there were a few bumps in the road to be negotiated. Some people figured that it was just the "cycling" that was "fishless" and added ammonia to tanks that already contained a few astonished fish.
Others thought that if 4 or 5 ppm ammonia was good, more would move the process faster, and they got stuck in a seemingly endless "nitrite spike." Remember, you're adding only just enough ammonia each day so that the result is brought back to 4 or 5 ppm. This requires some daily testing; if you were automatically to add 4 or 5 ppm ammonia each day, you might be shocking the ammonia-sensitive nitrifiers with a build-up of free ammonia.
Sources of ammonia. Household "ammonia" is a fairly dilute solution of ammonium hydroxide with some "quality control" agents. To avoid the perfumes, sudsing agents, surfactants and dyes in consumer-type ammonia cleaning products, some of which are lethal to fishes, look for a generic brand from a hardware store, such as Ace Hardware's Janitorial Strength Ammonia, a 10% solution of ammonium hydroxide in water. But don't hunt for a stronger solution: if you go above about 5 ppm ammonia at any point, you'll only delay the completing of your "cycle."
I raised some eyebrows when I revealed in a web forum that I hadn't been ransacking my neighborhood in a search for a pure non-sudsing ammonia with no added dyes and perfumes, when I could always produce at least a few drops of a completely natural, though more personal, source of ammonia. The truth is, long before the nitrating cycle was community knowledge, I have always begun new planted aquaria by adding a few tablespoons of my urine. Good grief! The uproar at AquariaCentral sent me back to the books. I soon found out that urine actually has a very small NH3 content. Urine has a specific gravity of about 1.017-1.020, owing to its dissolved solids, about 60% of which are organic substances. Besides ammonia, those organics include urea, uric acid, and creatine, which are all bacterially decomposed to form carbon dioxide— and more ammonia. The other 40% of the dissolved solids in urine are inorganic NaCl, (the "salt" content), K, PO4 and SO4. Frankly, it all sounds to me like stuff you'd be adding anyway. There are no bacteria in healthy urine.
Nevertheless, people at the forum were more appalled than I could ever have expected. Some expressed their squeamishness in terms of "dangers." As far as I could tell, the greatest danger in this technique is of falling off the stepladder (heh heh heh). I was disappointed that no one asked how I had the control to fill just one tablespoon!
The "nitrite spike." In fishless cycling, a lot of people seem to get fixated on the "nitrite spike." What's that? After a few days of adding enough ammonia to bring it back to the setpoint of 4 or 5 ppm in your daily tests, you'll find that ammonia-metabolizing bacterial populations begin to produce appreciable quantities of nitrite (NO2). Soon the nitrite-respiring bacteria (Nitrospira type, as it now appears) catch up, however, and nitrite levels then drop to zero. If you plotted the NO2 on a graph over the days, it would "spike." Test results of zero ammonia/ammonium (NH3/NH4) and equally undetectable nitrite (NO2), usually with the appearance of some low levels of nitrate (NO3), signify that "cycling" is done: the "cycled" aquarium is now ready to receive fishes.
This passing "nitrite spike" is not necessary: it's nothing but an artifact created by the slower reproduction rates of the nitrite-users, compared to the bacteria that do the first-stage metabolizing of ammonia. And some folks are suggesting that perhaps the very presence of ammonia itself has a repressing effect on these nitrite-metabolizing bacteria, so that they aren't able to establish effective populations until the ammonia-metabolizing bacteria have stabilized and reduced the ammonia to low levels. Nitrite-oxidizing bacteria do seem to be more sensitive than their ammonia-oxidizing partners. In fact, under stressful conditions some nitrite can reappear, even in a thoroughly established aquarium.
So, are Nitrospira and their ilk sensitive to ammonia? Could be. I'm not equipped to run an experiment, but I can see that you'd have to set up some identical aquaria and introduce nitrite (but not ammonia) to each of them, until tests began to register some nitrate. Then you'd have to add ammonia to half the tanks, while keeping up the diet of nitrite. Perhaps you'd find that the tanks with added ammonia showed reduced nitrite-to-nitrate metabolism. Certainly the nitrite-respiring bacteria are sensitive to low temperatures and low pH. You see why, once ammonia levels have dropped to zero, you could effectively "re-seed" with mature filter media or gravel taken once again from an established tank, just as you did at the outset, and thereby reduce the duration of the "nitrite spike" in a cycling tank.
About "cycling" with bacteria-in-a-jar products. "Cycling" a new aquarium is easy. It's inevitable, in fact, if you'll give it time. Nitrifying bacteria are so apt to scavenge any source of nitrogen — whether in the form of ammonia or as nitrite — that it takes some pretty good lab technique to keep suitable cultures free of them. An aquarium with fish in it, even without plants, is a very suitable culture medium. A lab technician could tell you better than I, how nitrifying bacteria creep in and "contaminate" many delicate experiments. A whole chapter narrating some adventures with the disconcerting results of these bacterial "ghosts and wraiths" features in John Postgate's The Outer Reaches of Life, available in paperback. This book is too good to miss! And it's full of material that illuminates the material that will definitely be on the mid-term!
"Cycle" and similar products are scorned by about half the fishkeepers who post on the Web, perhaps in reaction to Hagen's upbeat but highly generalized advertising ("Cycle keeps aquariums healthy... maintains an optimal biological balance" — that kind of thing). And maybe some folks bristle at the manufacturer's suggestion to keep on adding the product to an established aquarium. If such products do establish an optimal biological balance, why do they have to be constantly added? is usually the question. Investing Cycle with an air of scientific scrupulousness, Hagen posts at its site a document quite irrelevantly assuring us that the product's bacterial strains aren't contaminated with any of a list of often-pathogenic bacteria. In a debating club this would be called a "red herring." You'll see immediately that the contamination of the manufactured product has never been the question. I can't dismiss "Cycle" and its brethren; you see, it would be very difficult to demonstrate that the nitrifying bacteria were not already there, dormant somehow, freeze-dried perhaps, right in the jar, all the while, as the marketers claim! If "Cycle" does contain ammonia (test for ammonia yourself, using 10ccs of distilled water and a few drops of Cycle) ostensibly it is to feed the dormant bacteria, as their consumer relations desk claims. Some nitrifying bacteria inevitably cling to aerosols and airborne flotsam and jetsam, ready to colonize any wet source of ammonia open to the air. Certainly they do invariably "appear" in any culture of those ammonia-containing "cycling" products sold by your LFS.
Nevertheless, nitrifying bacteria also eventually do appear in any ammonia-bearing culture not containing these products, too! The bacteria-in-a-jar products all work. So would a crumbled oatmeal cookie!
Bio-Spira. Dr. Tim Hovanec and a team at Marineland Labs developed a product named Bio-Spira, put on the market in 2002, which was said to contain a founder population of Nitrospira-type bacteria to start the "cycle." Several years of Hovanec's DNA sequencing and bacterial culturing had identified the particular strains of Nitrospira responsible for metabolizing ammonia to nitrate, before this product was put on the market. Retailers' initial resistance to this product, which needed to be kept under refrigeration and had a limit to its shelf life (about a year), resulted in a change of formula: remarketed by Marineland/Tetra in 2008 as Safestart, the new formulation was presented as stable on the shelf. The AquariumWiki cued me in that the same patent covers the bacteria in SafeStart. In the meantime Hovanec withdrew to form his own company and distribute his own blends of bacteria, one for freshwater and another for saltwater, his One & Only Nitrifying Bacteria, with a labeled date code on each bottle.
By the way, if you'll plant your tanks from the very start, most of these "cycling" problems will disappear.
There's more on the subject of nitrifying bacteria (and anaerobic de-nitrating bacteria too) in the context of the nitrogen cycle in established aquaria.