Fish Tank Cycling Time Calculator

“Cycling” a fish tank means establishing the nitrogen cycle, and it is one of the most important ideas in fishkeeping. Without it, fish die of ammonia poisoning within days in a new tank. With it, the same aquarium can safely house fish for years.

The nitrogen cycle explained: Fish produce waste containing ammonia (NH₃/NH₄⁺). Ammonia is highly toxic. In a cycled aquarium, beneficial bacteria convert ammonia in two steps:

1. Nitrosomonas bacteria convert Ammonia to Nitrite (NO₂⁻) (Nitrite is also highly toxic)

2. Nitrospira bacteria convert Nitrite to Nitrate (NO₃⁻) (Nitrate is far less toxic, removed with water changes)

How long does cycling take? Fishless cycling with pure ammonia: 3 to 6 weeks The rotting-food method: 4 to 7 weeks, slower because the ammonia arrives unevenly With bacterial supplements (Tetra SafeStart, Fritz TurboStart and similar): 1 to 2 weeks With seeded media from an established tank: 1 to 2 weeks, the fastest route there is Fish-in cycling: 4 to 8 weeks, and every one of those weeks is hard on the fish

Cold water, a marine or reef setup, and a filter with little bio surface all stretch these out. The worst combination this calculator will return is 8 to 12 weeks. If you are past three months with ammonia still climbing, something else is wrong: check that you have not been knocking the colony back with water changes or medication.

Factors affecting cycling speed:

• Temperature: Bacteria are most active at 26 to 30°C (79 to 86°F), which is what the Warm option below covers. Below 20°C (68°F), cycling slows dramatically. Turn the heater up while the tank is cycling.
• pH: Bacteria prefer pH 7 to 8. Very acidic water below 6.5 inhibits bacterial growth.
• Ammonia source: Pure ammonia with no surfactants is the cleanest source. Alternatively, add a pinch of fish food and let it decompose daily.
• Surface area: Bacteria colonize surfaces, so sponge filters, ceramic media and bio-balls give them far more to work with than smooth plastic or glass.
• Existing media: Adding a handful of gravel or a piece of filter media from an established tank dramatically accelerates cycling.

How much ammonia to add

The target for a fishless cycle is 2 to 4 ppm, which for most people means 3 ppm and then testing. One ppm is one milligram of ammonia per liter of tank water, so a 100 liter tank needs 300 mg of ammonia to reach 3 ppm.

Household ammonia is sold at anywhere from 3% to 10% by weight, and the bottle rarely makes that easy to find. At 10%, one milliliter carries roughly 100 mg of ammonia, so that 100 liter tank needs about 3 mL. At 5% it needs twice as much. The calculator works this out for whatever strength you enter.

Two cautions that matter more than the arithmetic. Only use ammonia with no surfactants, no perfume and no detergent: shake the bottle, and if it foams, do not put it in the tank. And dose half, test, then top up, because bottle strengths drift with age and an overdose above about 8 ppm stalls the very bacteria you are trying to grow.

Cycling milestones:

• Week 1 to 2: Ammonia rises as bacteria have not established yet
• Week 2 to 3: Ammonia peaks, and nitrite begins to appear as Nitrosomonas establish
• Week 3 to 5: Nitrite peaks, and nitrate begins appearing as Nitrospira establish
• Cycle complete: Ammonia and nitrite both read 0 ppm, and nitrate is detectable

Signs of a completed cycle: Use a liquid test kit rather than strips, which are not accurate enough to trust here. You are looking for ammonia at 0 ppm, nitrite at 0 ppm, and nitrate above 5 ppm. When 2 to 4 ppm of added ammonia is gone within 24 hours and nitrite is still 0, the tank is fully cycled.