Guide to tank cycling

CYCLING A PLANTED TANK

Biofiltration in a planted tank

Simple summary of cycling a planted tank:

Fish & plants excrete organic waste; a portion of it is broken down into toxic ammonia
Bacteria & archaea in the filter & substrate converts ammonia to nitrite (Nitrosomonas and Nitrosococcus )
Different strains of microbes converts nitrite to nitrate (Nitrobacter and Nitrospira)
Nitrate levels accumulate in the tank; nitrate can be absorbed by plants or removed by water changes. In a well planted tank, plants can & will utilize ammonia directly, by passing the cycle.
Cycling a planted tank means building up these colonies of bacteria before adding livestock. This is so that any toxic ammonia produced by livestock can then be processed swiftly into less harmful nitrates.
Ammonia cycling is just one form of organic waste in the tank. However, it is the one that affects livestock and algae the most, so tank cycling's first goal is to get ammonia cycling done.
Algae feeds off organic waste. Hence, taking care of the tank's microbial community is important, as these microbes break down organic waste (such as sugars and proteins excreted by plants) and make it unavailable to algae. It takes time for a tank to be biologically mature (weeks), even though ammonia cycling itself may be done quickly.

what about planted aquarium substrate?

Planted aquarium Substrate houses a large amount of beneficial bacteria. This is especially so for active substrates with organic matter. However, there is generally very low rates of water flow through substrate layers (unless one uses an under-gravel filter). Substrates still act as a capture point for fine particles; bacteria bio-films bind fine particles together and the substrates act as a housing ground for particulate matter to settle.

HOW TO CYCLE A TANK

The old way

The old method of cycling a planted tank was to add a light fish load of hardy fish, which then produce organic waste and ammonia; the tank was then left to build up the bacteria naturally over time. This can take many weeks, as more and more livestock was added slowly. Seeding the tank with seasoned filter media or mulm from the substrate of a previous setup are additional ways to kick start the cycling process. Depending on how it's done, this process can be tough on livestock.

Today's fish-less cycling with plants

Nowadays, fish-less cycling with plants is the rage. This involves adding liquid ammonia regularly into a new tank; fully setup except for livestock or plants, to grow the bacteria colonies.

A dose of 4ppm of ammonia is added, then the water is tested after a couple of days (takes about 3 days or so from a cold start to even see levels change). When ammonia levels start to decline, one would start seeing a build-up of nitrite. The first step of the ammonia oxidation cycle is started (but not necessarily completed).
Additional ammonia is then added every day to feed the bacteria (add enough to raise levels back to around 4ppm). After many more days, nitrite levels would fall, and one can then measure nitrate levels rising as the bacteria converting nitrite to nitrate has populated. This take longer than step 1 as the bacteria responsible for nitrite conversion to nitrate populates more slowly.
Eventually, even with additions of 4ppm of ammonia, nitrites and ammonia would measure at 0 after a 12hr period, while nitrates accumulate. The ammonia cycling process for the tank is now complete. The entire process can take 4-6 weeks depending on tank parameters.
Change tank water to reduce nitrates before adding livestock.
Adding starter bacteria cultures at step 1 greatly speeds up the process.

Cycling with ADA aquasoil / other ammonia rich aquasoils:

If you do not use starter bacteria products, it is a good idea to add mulm or used filter media to kickstart the cycle. Without the use of starter bacteria products, full cycling of the tank can take up to a month or more. The lowered pH of the tank water due to the buffering capacity of ADA aquasoil reduce ammonia toxicity as most of the ammonia exists as less toxic ammonium(NH4+) format in low pH (below 7). This allows hardy plants to be planted and grown even in early stages where ammonia levels are detectable. However, more sensitive plants, such as Utricularia gramminifolia, and tissue culture plants in general, should only be planted after the tank is cycled. On the flip side, if the aquasoil lowers the pH lower than pH6, bacteria reproduction is affected.
With the use of starter bacteria products, cycling time can be shortened to a week plus. After filling the tank, I like to let the tank soak without filter for a couple of days without a filter. On the second or third day I'll do a 100% water change then start running the filter. This removes organic debris, dust and sugars released from the woods and prevent the filter from taking up a lot of debris at the start. After starting the filter, I'll dose the starter bacteria culture into the filter intake. I'll increase the KH to 3dKH and add a few pieces of limestone (about 200 grams of Seiryu per 100 litres) temporarily to the tank to prevent tank water from becoming too acidic.
Wait 4 days and take ammonia and nitrate readings. If there nitrate readings, it means that the cycling has started. However, due to the tremendous amount of ammonia the aquasoil releases, chances are ammonia readings will still be high. I'll do a 90% water change and dose another dose of starter bacteria culture.
Wait a day and take ammonia/nitrate readings. If ammonia reads 0 and nitrates reading is positive, tank has cycled. However, if soil load is high, ammonia reading can still be positive as the soil releases a lot of ammonia, in that case, I'll wait a couple of days and repeat step 3. Due to reduced ammonia toxicity below pH 7, which is what will generally happen in aquasoil tanks, planting of standard, non-delicate plants can happen as long as ammonia readings are low (below 1ppm or so). However, for livestock I think it is better to wait till the tank has fully cycled.

using starter bacteria culture

Without a starter bacteria culture, cycling can still be slow. One can use the old method of using old filter media or mulm from tank to kickstart the cycling.

Using bacteria products available on the market, such as API Quick Start work (surprisingly) well, allowing an almost instant start. Bacteria products used to be touted largely as snake oil, but my tests on API's version of starter bacteria culture worked well - cycling an ammonia rich aquasoil tank in a few days. Some other brands on the market have not proven as effective though.

Raising the tank temperature to 28-36 degrees Celsius (82-97f) also speeds up bacteria reproduction. Below a pH of 6, bacteria activity slows and eventually stalls - they operate more quickly above pH 7. Baking soda/sodium bicarbonate or potassium bicarbonate can be added to a tank to raise the KH/pH levels if it is too low (to 3 to 5 dKH). Alternatively, addition of small amounts of coral chips/limestone to the tank environment can also raise the KH/pH.

However, if for some reason you already have livestock in the tank, ammonia toxicity is much lower when the water is acidific (below pH 7), as most of the ammonia exists in ammonium (NH4+) format. So keeping the water acidic actually prevents livestock from being poisoned by ammonia. In this case, maintaining a pH between 6.5 and 7 is a middle ground where bacteria can still multiply while fish are protected somewhat by the low acidity.