Cycling a planted aquarium: the first weeks and when to add fish
Updated
Cycling is the period when bacteria that process fish waste settle into a new tank. Until it is done the water looks clean but is dangerous for livestock.
What happens during cycling
Decaying food, dead leaves and leaching aquasoil produce ammonia (NH₃/NH₄⁺). Nitrifying bacteria turn it into nitrite (NO₂⁻) and another group turns nitrite into nitrate (NO₃⁻). Ammonia and nitrite are toxic to fish and shrimp; nitrate at normal levels is a nutrient for plants.
The bacteria live mostly in the filter, the substrate and on every surface. That is why changing all the water does not speed cycling up – the water carries almost none of them.
How long it takes
Usually four to six weeks. It is faster with filter media from an established tank and slower in cooler water or without a source of ammonia.
Aquasoil releases a lot of ammonia for the first two or three weeks. That feeds the bacteria, but it is also why you change water often at the start.
What to test
- NH₄ and NO₂ – once or twice a week. Cycling is done when both read zero for several days in a row.
- NO₃ – a rise shows the second group of bacteria is working.
- pH and KH – as a baseline; aquasoil usually lowers KH.
Test strips are fine for orientation; liquid tests are more reliable for NO₂ and NH₄.
Water changes while cycling
With aquasoil a common routine is 50 % every second or third day in the first week, then twice a week and from week three or four once a week. The changes keep ammonia low so algae have less to work with, and they do not hurt the bacteria.
With an inert substrate (sand, gravel) less is needed, typically 30 % once a week.
Plants and light from day one
Plant densely at setup. Fast-growing stem plants and floaters take nutrients away from algae. Start with six hours of light a day and add half an hour each week up to seven or eight hours.
If you run CO₂, start it from day one – stable CO₂ prevents algae better than anything else.
When to add livestock
Only once NH₄ and NO₂ read zero for several days. Otocinclus, Amano shrimp and nerite snails are good first inhabitants – they help with the algae that almost every new tank gets. Add fish gradually in smaller groups so the bacteria can keep up with the load.
Common mistakes
- fish in the first week “to cycle the tank”,
- full fertilizer doses in the first weeks with aquasoil that already releases nutrients,
- a long photoperiod from day one,
- rinsing the filter under the tap – chlorine kills the bacteria,
- changing several things at once when algae appear.