I am not going to discuss how the nitrogen cycle works……that is another discussion entirely, & if you are a novice to fish-keeping & do not know what the terms “cycle a filter” or “nitrogen cycle” mean, please stop reading this, find the information on that subject  & familiarize yourself w/ this most important aspect of the aquarium hobby.

When people first think about keeping tropical fish, they often go to their local aquarium/pet store, & ask “How do I cycle my fish tank?” Well, that is a misnomer…while all types of bacteria certainly will inhabit just about every square inch of your aquarium, they will not colonize or reproduce in sufficient quantities to handle the waste load your fish produce.  In point of fact, you are not cycling the aquarium……what you need to do is establish a colony of two types of bacteria in a biological filter, which will have some sort of medium that they can easily attach to & reproduce. This is what I term a “dedicated” biological filter, one whose (almost) sole purpose is to house BB (beneficial bacteria) that will break down the ammonia (& then nitrite) produced by your fish & then by the bacteria that eat the ammonia. 

Good dedicated biological filters come in many forms……the best you can have is a “wet/dry” (sump) filter, one that utilizes an overflow of water  from the tank, which then flows over a layer of media, such as bio-balls, ceramic noodles, etc. that have a constant film of water supplied to them from the aquarium…these then grow BB on them, so they get a food source of ammonia & nitrite, & are also able to obtain atmospheric oxygen, rather than compete w/ all other organisms in the aquarium for available O2….the BB will reproduce much faster & you will have a larger colony than in some other biological filters that depend on the O2 from your aquarium water, such as canisters, which have submerged bio-media.

Canister filters are a good choice also….they can be fitted w/ all sorts of not only bio-media, but also mechanical filtration to remove solid waste from the aquarium. Canisters are also pressurized, so there is no “backflow”, like there is in “hang on back” (HOB) filters. The aquarium water must pass through all media in the canister before it returns to the aquarium. 

There are also HOB filters that can do a good job of biological filtration…some even have a “wet/dry” component built into them. I DO NOT recommend you rely on filter cartridges for biological filtration, especially if you intend to keep cichlids of any size. These cartridges are inefficient at best, & even small cichlids demand pristine water. 

Now that we have briefly discussed filtration, let’s describe methods for cycling filters:

  1. Some people advocate using bottled ammonia as the “food source” for starting the nitrogen cycle. I DO NOT. While it does work, pure ammonia is a dangerous chemical, & look @ it this way…if you splash ammonia in your eyes, you will take a trip to the hospital. If you splash aquarium water in your eyes, you simply have to wash it out. People also advocate this method as they believe it is “cruel” to the fish being utilized……I can assure you that if done properly, nothing could be further from the truth.
  2. There are some who utilize pieces of fish, shrimp, flake food, etc. introduced into the aquarium water as the ammonia source. Not only does this not work, it will introduce fungi & dangerous bacteria into your water column, possibly infecting you. ABSOLUTELY NOT RECOMMENDED.
  3. The method I have used for decades & the one I always recommend is referred to as “Fish-in” cycling. This means you utilize live fish as your ammonia source. In my experience, this is the easiest & most natural way, & I believe it produces a much stronger cycle than any other method. Also, as long as you use hardy species for this purpose (I always use Giant Danios or Platys, tough fish)that can withstand elevated ammonia/nitrite levels, monitor ammonia/nitrite levels w/ a quality test kit, & perform small partial water changes, you will never lose a fish. I have also recommended this method to countless people, & they were amazed @ how easy it is.
  4. There are also those who use “instant cycling” or “bacteria in a bottle” products. I can assure you that they are worthless…spend your money on something worthwhile, like a quality test kit.
  5. To do the “fish-in” method, simply purchase a hardy species of fish…..the best species IMO is Devario aequipinnatus, the Giant Danio. Inexpensive to purchase, very hardy, & actual;ly a great “dither” fish for cichlids…..I use 2 for every 10 US gallons……drop them in the aquarium, feed them sparingly, do as described above, & then in 10-12 weeks, you should have a well-cycled filter. 
  6. OLD SCHOOL WORKS!!