The dark start is exactly as it sounds: cycling an aquarium in the dark, with no plants and no lights, before anything goes in. It asks for patience up front and pays it back with an aquarium that starts clean, cycles itself with almost no intervention, and skips the algae bloom that catches out most new planted tanks. We recommend it to anyone setting up a planted aquarium.
Why it works
A new planted aquarium is at its most vulnerable in the first month, and for one reason: it is full of ammonia and other nutrients before anything is established enough to use them.
Aquasoils are the main source. Most of them are loaded with organic nitrogen and leach substantial ammonia for the first few weeks of use. That is by design, and it is what makes them such good substrates in the long run. But in those early weeks the ammonia has nowhere to go. The biological filter has not established yet, and newly planted stems have not rooted or begun growing properly, so they cannot take it up either.
What can use it immediately is algae. Algae spores are already present on hardscape, in tap water and in the air, and they need three things: nutrients, water and light. A newly planted tank with fresh aquasoil supplies all three at once, which is why so many first scapes go through a green haze and a bloom of hair algae in the opening weeks.
A dark start removes the third one. Nutrients and water are there, but with no light, algae cannot establish. The soil leaches out, the bacteria colonise the filter, and by the time you switch the lights on, the ammonia is gone and the aquarium is ready.
What you get out of it
That last one is underrated. A conventional planted tank start means daily water changes for the first fortnight to keep ammonia down. A dark start needs none of that, because there is nothing living in the tank to protect. You let the ammonia sit there and feed the bacteria, which is exactly what it should be doing.
How long it takes
A typical dark start. Ammonia peaks first, then nitrite, then the tank is cycled. Seeding the filter with bacteria shortens the dark period considerably, though with aquasoil you are also waiting for the soil itself to finish leaching.
Four to six weeks is the usual range with aquasoil. With an inert substrate and a dosed ammonia source it can be much shorter, because you are only waiting for the bacteria rather than for the soil as well.
Do not rush the ending. The point of a dark start is to plant into a finished aquarium, and switching the lights on while ammonia is still present gives up most of the benefit.
How to dark start an aquarium
Fill your aquarium with your chosen substrate and set your hardscape. Make sure wood is waterlogged or attached to rocks so it does not float when the tank is filled.
Get the layout right now. Rescaping later means disturbing an established substrate and pulling the tank apart, so this is the moment to be fussy about it.
Chlorine and chloramine are added to tap water specifically to kill bacteria, and they will kill the colony you are trying to grow.
Treat the water with AquaLabs Vitality. Neutralisation happens within seconds, so there is no waiting period before the next step.
Both run continuously for the whole dark start. Set the heater to 26 to 28 °C, because nitrifying bacteria establish considerably faster in warm water and there are no fish to consider yet.
Fill the filter properly while you are at it. The colony grows on surfaces, so the amount of media you give it sets the size of the biological 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.
The bacteria need something to feed on. What you do here depends on your substrate.
Using aquasoil? You do not need to add anything. Most aquasoils release ammonia for the first weeks of use, and that is the whole reason a dark start suits them so well.
Using an inert substrate such as gravel or sand? Nothing will produce ammonia on its own, so you need to dose it. Use an aquarium-specific ammonia additive rather than household ammonia, which often contains surfactants and perfumes.
AquaLabs Fishless Start is an ammonium chloride solution with added essential nutrients to support the developing bacterial colony. Dose to 2 ppm and top it back up whenever it reaches zero.
Optional, but it is the difference between a dark start measured in weeks and one measured in months. Without seeding you are waiting for nitrifying bacteria to arrive by chance and multiply from nothing.
AquaLabs Nitro Cycle combines fast-growing heterotrophic bacteria with true nitrifying autotrophs, which colonise the filter media and form the permanent biological filter.
No lighting, no fertiliser, no CO2. There are no plants to feed and nothing that needs light. Ambient room light will not cause problems, but keep the tank out of direct sunlight, which is bright enough to give algae the opening you are trying to close.
Test ammonia, nitrite and nitrate every few days to follow the cycle. Daily testing is not necessary here, because there is nothing at risk.
The AquaLabs Master Test Kit covers ammonia, nitrite, nitrate, pH and KH, which are the readings you will want across the whole dark start.
Nitrification consumes alkalinity, so KH falls steadily as the cycle runs. Most aquasoils actively strip it as well, which makes a soil-based dark start the most likely of all setups to run out.
If KH reaches zero, pH crashes and the cycle stalls. Test it every few days rather than only at the start.
Top up carbonate hardness with AquaLabs KH Plus if it drops away during the dark start.
When ammonia and nitrite both read zero and nitrate is present, the aquarium is cycled.
Nitrate will be high after weeks of converted ammonia, so do a large water change before planting, and dechlorinate the replacement water. Then plant the tank, start your fertiliser and CO2 routine, and switch the lights on.
Begin with a short photoperiod, around six hours, and extend it over the following weeks as the plants settle in. A cycled tank is far more forgiving than a new one, but there is still no reason to hand algae a twelve hour head start on day one.
Things worth knowing
Common questions and mistakes
- Can I plant it and then dark start? No. Plants need light, and a month in the dark will kill most of them. The point is to plant afterwards, into a finished aquarium.
- Do I need to black the tank out completely? No. Ambient room light is fine. Direct sunlight is not, so move the tank or cover that side if it catches the afternoon sun.
- Do I need CO2 or fertiliser? No. There is nothing to feed. Save both until you plant.
- Should I do water changes? Generally not. The ammonia is the food supply, and removing it slows everything down. The exception is if a very heavy soil leach pushes ammonia far higher than about 4 ppm, which can inhibit the bacteria you are growing.
- Can I use household ammonia? Not recommended. Many contain surfactants or perfumes. Use an aquarium-specific product.
- Do not clean the filter during the dark start. You would be removing the colony you have spent weeks building.
- Do not add livestock early. There is no such thing as a fish that will speed this up, and the whole advantage of a dark start is that nothing is exposed while it runs.
Temperature, pH, KH, oxygen and surface area all affect how long a dark start takes. They are covered in more detail in how to cycle an aquarium faster, and if you would rather cycle a tank you are planting straight away, see how to fishless cycle an aquarium.
The short version
- Lay out substrate and hardscape, and get the scape right first time
- Fill with water dechlorinated using Vitality
- Run the filter and heater continuously at 26 to 28 °C
- Aquasoil supplies its own ammonia. For inert substrates, dose Fishless Start to 2 ppm
- Seed the filter with Nitro Cycle to shorten the wait
- Lights off, no fertiliser, no CO2, no water changes
- Test ammonia, nitrite, nitrate and KH every few days
- Top up KH with KH Plus if it drops away
- Cycled means ammonia and nitrite at zero with nitrate present
- Large water change, then plant, dose, and start with a six hour photoperiod
A dark start asks you to look at an empty tank for a month, which is the hardest part of it. What you get in return is a planted aquarium that starts cycled, starts clean, and skips the algae battle that most new scapes spend their first two months fighting.