A fish-in cycle builds the biological filter in an aquarium that already has fish living in it. It is a completely valid way to cycle a tank, and countless aquariums have been started this way. It simply asks more of you than a fishless cycle does, because you are managing water quality daily rather than letting the tank get on with it.
When you will be doing this
Your fish are already in the aquarium. That is the situation this guide assumes throughout, and it is a common one:
- Fish were added to a new tank before anyone explained cycling
- A filter failed, or was cleaned too thoroughly, and the colony is gone
- Medication wiped out the biological filter in a stocked aquarium
- You have taken on fish at short notice and have nowhere else to put them
- You simply prefer to start this way
If the tank is still empty and you have not bought fish yet, we would recommend a fishless cycle instead. It reaches the same endpoint without asking anything of your fish, and it lets you stock the aquarium fully the day it finishes. But if the fish are already in, a fish-in cycle done properly works perfectly well.
What your fish experience
Fish are not sitting in ammonia and nitrite for the whole cycle. What actually happens is a series of short rises: ammonia builds up over a few days, you change water and it drops back to safe levels, and it builds again. Later in the cycle the same pattern repeats with nitrite. Between those rises, water quality is fine.
Your job is to keep every one of those rises short and low. That is what daily testing and prompt water changes are for, and it is why this method works.
Ammonia damages gill tissue, and the damage accumulates. Nitrite works differently: it binds to haemoglobin and prevents the blood carrying oxygen, which is why fish in a nitrite spike gasp at the surface in water that is perfectly well oxygenated. Both are readily survivable at low concentration for short periods, which is exactly the situation you are managing.
Why pH changes everything
This is the part most guides leave out, and it matters more than almost anything else you will read about fish-in cycling.
What a test kit calls "ammonia" is really two things in equilibrium: free ammonia (NH3), which is highly toxic, and ammonium (NH4+), which is far less so. The split between them is set by pH. The higher the pH, the more of your total ammonia exists in the dangerous form.
The same 1 ppm reading on your test kit is roughly 250 times more dangerous at pH 8.5 than at pH 6.5. Temperature has the same effect in the same direction, though far less dramatically.
Two practical consequences follow from this.
First, a soft, acidic aquarium is a much more forgiving place to run a fish-in cycle than a hard, alkaline one. If you keep African cichlids or a marine tank, treat every ammonia reading as more serious than the same number in a soft water community tank.
Second, and this catches people out: do not raise pH during a fish-in cycle. Adding buffers or alkalinity while ammonia is present converts harmless ammonium into toxic free ammonia, and can affect fish that were coping fine an hour earlier. Get the ammonia down first, then worry about pH.
The method
Every day the cycle takes is another day of managing water changes around your fish, so this is the first thing to do rather than the last. Adding an established bacterial culture is the difference between a cycle measured in days and one measured in months.
AquaLabs Nitro Cycle combines fast-growing heterotrophic bacteria, which begin drawing ammonia down straight away, with true nitrifying autotrophs that colonise the filter and build the permanent biological filter.
Daily testing is not optional here. With fish in the tank, your test results are the early warning that lets you act before a rise becomes a problem.
Test at roughly the same time each day so the numbers are comparable, and write them down.
The AquaLabs Master Test Kit covers ammonia, nitrite, nitrate, pH and KH. Test strips are not precise enough for this job, because the difference between 0 ppm and 0.25 ppm is the difference between doing nothing and doing a water change.
Test both every day and treat them as two separate triggers. If ammonia reads 0.25 ppm or above, change water. If nitrite reads 0.25 ppm or above, change water. Either one on its own is enough, and you do not wait for both.
Change 25 to 50 percent depending on how high the reading is. Match the temperature of the new water to the tank, and dechlorinate it before it goes in.
Water changes slow the cycle slightly, because you are removing the ammonia your bacteria feed on. That trade is worth making every time.
Treat all replacement water with AquaLabs Vitality. Chlorine and chloramine kill the bacteria you are trying to establish, and adding untreated tap water mid-cycle can set you back to the beginning.
Every gram of food that goes in becomes ammonia, whether a fish eats it or it rots in the substrate. Feed a small amount once a day, or every second day, and siphon out anything left after a few minutes.
Fish will not come to harm from a few weeks of light feeding. They are far better off than they would be dealing with the ammonia produced by overfeeding.
Nitrifying bacteria are aerobic, so good flow and surface agitation speed the cycle. It also helps your fish during the nitrite stage, when their blood cannot carry oxygen as efficiently as usual.
Add an airstone or angle the filter outlet to break the surface. It costs nothing and helps on both counts.
No new fish until the cycle finishes. No medications unless a fish is genuinely ill, because antibacterials do not distinguish between pathogens and your biological filter. No filter cleaning. No substrate deep-cleaning. No pH adjustment.
The aquarium is doing one job right now. Let it.
Make sure the filter is full of porous biological media and running continuously. The colony grows on surfaces, so the amount of surface you provide sets the size of the filter you end up with.
AquaLabs Nitro Nest is a highly porous biological media, engineered to carry the largest possible bacterial colony per litre of filter space.
What progress looks like
The final stage is the one to be careful about. A single zero reading is not proof of anything, particularly the day after a large water change. Look for several days of consistent zeroes while feeding normally before you call it finished.
Signs to act on straight away
Test results tell you what is happening before the fish do, which is the point of daily testing. If you see any of these, test immediately and change water regardless of what the last reading said:
- Gasping at the surface, or sitting directly in the filter outflow
- Rapid or laboured gill movement
- Red or inflamed gills
- Clamped fins, lethargy, sitting on the bottom
- Loss of appetite in a fish that was feeding well
- Brown or dark blood colour visible in the gills of pale fish, which suggests nitrite
What makes it slower
Avoid during a fish-in cycle
- Overfeeding. The most common cause of a fish-in cycle that never seems to end. Every uneaten flake becomes ammonia.
- Adding more fish. Each addition increases the ammonia load the colony has to catch up with, and extends the process.
- Cleaning the filter. You are cleaning away the thing you are trying to grow. Leave it alone until the cycle finishes, then rinse gently in old tank water only.
- Raising pH or KH while ammonia is present. This converts safe ammonium into toxic free ammonia. Deal with ammonia first.
- Medications and residual copper. These kill your biological filter along with whatever you were treating.
- Zeolite or ammonia-removing resin. It will lower your readings, but it also starves the colony, so the cycle never completes and you become dependent on replacing the resin. Activated carbon is fine and can stay in.
- UV sterilisers, ozone, protein skimmers and filter socks. Newly dosed bacteria are free-floating until they attach to a surface. Switch off, or remove socks, for 48 hours after dosing.
- Low temperature. Bacterial metabolism slows sharply in cool water. Within the range your fish tolerate, warmer is faster.
Temperature, pH, KH, oxygen and surface area all affect how long this takes, and they are covered in more detail in how to cycle an aquarium faster.
After it finishes
Test nitrate and bring it down with a water change if it is above 20 ppm, unless the aquarium is well planted and the plants have already taken it up.
Then add any further stock gradually rather than all at once. The colony is sized to the fish it has been feeding on, and a large sudden addition can produce a small secondary rise while it catches up. Add a few fish, wait a week, test, and repeat.
The short version
- Seed the filter with Nitro Cycle straight away
- Test ammonia and nitrite daily with the Master Test Kit and record the results
- Change 25 to 50 percent of the water whenever ammonia reaches 0.25 ppm, or whenever nitrite reaches 0.25 ppm
- Dechlorinate every drop of replacement water with Vitality
- Feed lightly and siphon out anything uneaten
- Keep oxygen and surface agitation high
- Do not raise pH or KH while ammonia is present
- Add no fish, no medications, and do not clean the filter
- Cycled means several consecutive days of zero ammonia and zero nitrite with normal feeding
- Add any further stock gradually, a few fish at a time
A fish-in cycle asks for a test kit, a few minutes each day, and a bucket on standby. Test daily, change water on the rule rather than on a hunch, and keep the feeding light, and your fish will come through it fine.