Fast Aquatics

What is the nitrogen cycle?

Fish, invertebrates, and uneaten food produce ammonia (NH3 / NH4+). Ammonia is toxic to fish at concentrations as low as 0.25 ppm. In a healthy aquarium, two groups of bacteria break ammonia down sequentially:

  1. Nitrosomonas bacteria convert ammonia to nitrite (NO2-), which is also toxic
  2. Nitrospira and Nitrobacter bacteria convert nitrite to nitrate (NO3-), which is far less toxic and removed via water changes or denitrifying processes

"Cycling" is the process of growing populations of these bacteria large enough to handle your tank's bioload. A tank without an established cycle cannot safely house fish.

The 60-day rule: 90% of fish deaths in a new tank trace to incomplete cycling. If your tank is less than 60 days old and your fish are dying, test your water before assuming disease.

How long does cycling take?

4-8 weeks for a freshwater tank. 6-10 weeks for saltwater. Faster with seeded media (substrate or filter material from an established tank). Significantly faster with bottled bacteria products like Dr Tim's One & Only or Fritz Zyme 7, but never instant.

Fishless cycling (preferred method)

  1. Set up tank with substrate, filtration, heater, lighting
  2. Fill with dechlorinated water (or RO/DI for saltwater + salt mix)
  3. Run filtration and heater for 24 hours to stabilize temperature
  4. Add ammonia source: pure ammonia (Dr Tim's Ammonium Chloride is standard) dosed to 2-4 ppm, OR a piece of raw shrimp left to decay
  5. Test daily for ammonia, nitrite, nitrate
  6. Watch the curve: ammonia rises and falls, nitrite rises and falls, nitrate accumulates
  7. When ammonia and nitrite both read 0 within 24 hours of dosing 2 ppm ammonia, the cycle is complete
  8. Do a 50% water change, add livestock gradually

This method takes 4-6 weeks but is the safest and most thorough.

Fish-in cycling (only if necessary)

Sometimes you inherit livestock without time to cycle. Fish-in cycling works but causes stress and risks death:

  1. Use a small number of hardy fish (zebra danios, white cloud minnows for FW; damsels for SW - though damsels become problematic later)
  2. Test ammonia and nitrite daily
  3. Water-change aggressively whenever ammonia or nitrite climbs above 0.25 ppm (typically 25-50% changes every 1-3 days)
  4. Continue until both read 0 within 24 hours of feeding
  5. Add additional fish slowly

This method takes 6-8 weeks and is harder on the livestock. Avoid if possible.

Seeding (cheat code, legal)

If you have access to an established tank, you can dramatically shorten the cycle:

  • Move filter media (sponge, ceramic media, biological balls) from established to new tank
  • Add a cup of established substrate
  • Use established tank water for water changes during the first month

The bacteria colonies transfer with the media. A seeded tank can cycle in 7-14 days. Combined with bottled bacteria, sometimes faster.

Common cycling myths

  • "My tank doesn't need to cycle, I have a sponge filter" - the sponge needs cycled bacteria too
  • "My tank is cycled because the water is clear" - clear water is not cycled water; test ammonia and nitrite
  • "I added bacteria in a bottle, I'm done" - bottled bacteria seeds the cycle, doesn't complete it; verify with testing
  • "I used water from another tank, I'm cycled" - water carries minimal bacteria; the colonies live on surfaces (substrate, filter media, decor)
  • "My LFS said it was ready" - test it yourself

What "cycled" looks like

A fully cycled tank shows:

  • Ammonia: 0 ppm
  • Nitrite: 0 ppm
  • Nitrate: 5-40 ppm (lower for reef, slightly higher OK for FW community)
  • Within 24 hours of dosing 2 ppm ammonia (fishless test) or feeding (with fish), readings return to 0

Maintaining the cycle

Once cycled, the bacteria sustain themselves on ongoing fish waste. The cycle can crash when:

  • You replace your filter media instead of rinsing it (always rinse in tank water, never tap)
  • You medicate with antibiotics that kill nitrifying bacteria
  • You add too many fish too quickly and overwhelm bacterial capacity
  • Power outage stops filtration for 24+ hours, killing aerobic bacteria
  • You change too much water at once with un-dechlorinated tap water (chlorine kills bacteria)

Test for 1-2 weeks after any of these events to confirm the cycle held.

Saltwater specific

Saltwater cycling adds a few wrinkles:

  • Live rock and live sand introduce bacteria that accelerate cycling
  • "Diatom bloom" (brown coating on glass and substrate) appears around weeks 2-4 - normal, passes
  • "Cyanobacteria bloom" (red slime) sometimes follows - manage with reduced lighting and flow improvement
  • Coralline algae appears as the tank matures - sign of stability
  • Reef tanks aim for ultra-low nitrate (1-5 ppm); FOWLR tolerates higher

The bottom line

Cycle your tank. Test your water. Don't add livestock until ammonia and nitrite are both 0 within 24 hours of feeding/dosing. The hobby's first lesson is also its most expensive when ignored.