Fishless Cycling is basically using alternatives to fish to introduce ammonia into the aquarium environment. The ammonia in general is intended to feed the developing bacteria needed to keep the fish alive once they are added. There are other applications for the methods and we will try to discuss a few of those as we go.
They all introduce varying levels of phosphate, which can increase the chances of having algae and diatom problems, or additional substances that increase the amount of dissolved organic matter in the tank needlessly. Filth in general is a breeding ground for bacteria including the types we don't want.
All of these are fairly difficult to regulate the quantity of ammonia produced. How much flake food to add to get a 5ppm reading of ammonia can vary from brand to brand.
This gets you started with what you want right up front. Saves time by having the food the bacteria need to develop in quantities enough to spur growth immediately. This is also measurable and more easily controlled. No additional additives are brought into the tank. No need to clean the substrate prior to adding fish.
Where fishless cycling is lacking at this point is in the areas of application. The original method can be used for all applications but in my opinion isn't the needed for all situations. Let's look at a few of the major applications and how they can have varied methods from the original fishless method.
First of all is the original method described in the handouts linked below by Nomad who has been a great help to me in learning about this topic. It basically consists of a spike with ammonia and follow up repeated equal spikes until no ammonia and nitrite is detectible. This method allows you to maximize the quantity of bacteria in the tank in quantities unattainable any other way. This is needed where the tank will be filled with fish to capacity or close to it afterwards. However if you will not be adding a maximum load of fish where a maximum quantity of bacteria will be needed there are faster shorter versions. This also scares me. Many people will run out and purchase an entire tank full of fish afterwards. The problem with this fishfull slamming after fishless cycling is if these fish are coming from multiple tanks or even multiple stores then the chances of you getting a large quantity of healthy fish from that many tanks or sources all at once is low. You can very easily find yourself having to nuke the bacteria bed you just struggled to create in the attempt to medicate a tank full of diseased fish. If your goal is the community tank mix involving many fish from multiple locations then quarantining new arrivals is in order and a shorter version of fishless will suffice.
This is basically adding ammonia to the tank one time to around 5ppm. What this does for you is it allows for a smaller bacteria bed to develop in a shorter period of time. It's not enough for a full load of fish but it will get you enough bacteria developed that the first batch of fish can be added fairly soon afterwards once the ammonia and nitrite is reduced to nitrate. Afterwards additional fish can be added safely at intervals once they have been through a quarantine tank and there are no health questions.
This method is also useful for testing planted aquariums prior to adding fish. Once the ammonia, nitrite, and nitrate are removed and the plants appear to be visually growing you should be able to add fish safely. In some cases you may never get a nitrite or nitrate reading if the plants remove the ammonia. If you do then you know you transported in some bacteria with the plants.
This method can also verify your shuffle the filter, gravel, water and media method from existing tanks transported in enough bacteria to safely add fish soon afterwards. The danger to this method is if you find yourself in a situation where the ammonia is used up completely but the nitrite spike remains protracted the bacteria that convert ammonia to nitrite go without food for the remaining time. In this case smaller repeated doses of ammonia will be needed to maintain the ammonia converting bacteria at their optimum levels while you await the development of the nitrospira.
More testing needs to be done on this particular method but it basically consisted of adding an enormous amount of ammonia on day one. Ten days later a large bed of bacteria had developed sufficient to crank out high nitrite and nitrate readings. A 100% water change followed and a full load of fish was added. No follow up ammonia additions were made. In many cases the water needs to be changed after the original fishless method anyway just to dilute the nitrate levels. Applications for this method can be to keep that hospital/quarantine tank cycled and ready for fish or fry when needed or in situations where you just don't have the time or ability to be home everyday for a month or more to fishless using the traditional methods. In my case I have store tanks that are emptied of fish as they sell out. A one time mega spike keeps the bacteria bed maximized for the next shipment for pennies and involves no additional time or thought except to do the 100% water change before I add new fish that I do anyway the day they arrive or the day before. A retail application is for those customers who cannot afford or refuse to purchase 3 test kits at once. If they at least get the nitrate kit then spike the tank heavy, once they are getting high nitrate readings the bacteria bed has developed to a degree that they can add their first fish after a water change of 100%.
More applications and variations will come to mind as time progresses. The idea is to plan ahead and base which methods you will use on the first occupants of the tank. Hopefully soon we'll have a readily available ammonia source available from the local fish store in your area and once we do we can begin to get a more uniform exact listing of the methods and how to implement them. Standardization in general leads to uniformity of results and predictability.