Inputs · Starting a system
Cycling a new system (fishless cycling)
A new system has no nitrifying bacteria yet, so the ammonia that fish give off has nowhere to go. Cycling means growing those bacteria before the fish arrive: one group turns ammonia into nitrite, a second turns nitrite into nitrate, which is what plants feed on. Feeding the young colony a measured ammonia source spares fish weeks of toxic water. Plan on about 3 to 6 weeks, and longer in cool water.1, 2, 3
Why you need it1, 2
Ammonia and nitrite are both poisonous to fish. FAO says tilapia and carp can show ammonia poisoning from 1.0 mg/L, and nitrite causes trouble from 0.25 mg/L. If fish are the only ammonia source in a new system, they sit through the ammonia peak and then the nitrite peak while the bacteria catch up; FAO says many new fish keepers lose their fish this way, which aquarists call new tank syndrome. SRAC 4502 calls stocking fish into an unstarted system the cold start: it can work, but it is slow and hard on the fish, feeding has to be cut back or stopped, and water has to be exchanged to keep ammonia and nitrite tolerable. Growing the bacteria first means less stress on the new fish, normal feeding from the first day and better water from the start (SRAC 4502).
How to use it1, 2, 4
1. Prepare the water. Run the pumps so water passes through the biofilter, use water with no chlorine left in it, bring it close to the temperature you will run at, and set pH, alkalinity and hardness to suit the fish you plan to stock (SRAC 4502). FAO adds: shade the fish tank and biofilter from direct sun before you start. SRAC 4502 finds pH 6.8 to 7.2 best during start-up. 2. Add ammonia a little at a time. FAO rates finely ground, fresh, disease-free fish feed as the safest source and the easiest to control. Household ammonia works only if it is pure ammonia, with no detergent, scent, colour or other additives. Manure is a strong ammonia source but can carry E. coli and Salmonella, so FAO says it must be sterilised first. As a guide, SRAC 4502 says 60 ml of clear 10 percent household ammonia raises 3,785 litres (1,000 gallons) by about 1.6 mg/L; add a little, let it mix, test, then add more. 3. Keep ammonia moderate. FAO's target is 1 to 2 mg/L: if it goes above 3 mg/L, change some water, because above 4 mg/L ammonia holds the bacteria back. (SRAC 4502 and the UF/IFAS guide HS1252 dose higher, 3 to 5 mg/L; see gaps.) 4. Seed bacteria if you can. A few arrive on their own from air, water and soil (FAO), but the process goes much faster with media or water from a working system (FAO; SRAC 4502 calls moved biomedia the most effective seed) or with a bottled culture (see Starter bacteria). FAO also describes priming media in advance by trickling a 2 to 3 mg/L ammonia solution over it for a few weeks. 5. Test and graph. Test ammonia, nitrite and nitrate every few days (FAO says every 2 to 3 days in one chapter and every 3 to 5 in another), from the same spot at the same time of day (SRAC 4502). Ammonia rises and falls first; FAO says the ammonia-eating bacteria form a colony 5 to 7 days after the first dose and nitrite starts rising 5 to 7 days after that; nitrite then peaks and falls as nitrate climbs. 6. Know when it is done. FAO defines the end as nitrite at 0, ammonia under 1 mg/L and nitrate rising steadily, about 25 to 40 days in good conditions. 7. Add fish slowly once ammonia and nitrite are both under 1 mg/L, and expect a smaller second spike; change water if either rises above 1 mg/L (FAO). 8. Plants: FAO says seedlings can go in once nitrate shows up, but expect slow growth and some temporary deficiencies; it recommends waiting 3 to 4 weeks for nutrients to build up. The UF/IFAS start-up plan in HS1252 runs: day 2 fill and set pH to 7.0 to 7.5; day 3 start pumps and dose household ammonia; day 4 add a bacteria culture; day 5 plants may go in; weeks 3 to 4 add fish once ammonia and nitrite are near zero. After start-up, HS1252 says never add ammonia or urea again, because the fish make enough. Temperature: SRAC 4502 says raising water 2 to 3 °C above the planned running temperature speeds things up, but no more, or bacteria die back when it drops.
In Florida2, 1, 5, 6
Many Florida city supplies use chloramine, which does not gas off like plain chlorine: Tampa's Water Department uses chloramine most of the year and Pinellas County switched to it in 2002. Start-up water must be free of residual chlorine (SRAC 4502), so treat city water for chloramine first (see Dechlorinating tap water). Temperature matters too: SRAC 4502's example start-up ran quickly at 28 °C (about 83 °F), FAO gives 17 to 34 °C as the working range for nitrifying bacteria, and FAO warns cool water can stretch cycling to two months. Our reading of those figures, not a source statement: a summer start will usually finish faster than a winter start in North Florida.
Cautions2, 3, 7, 1
Do not stock fish while ammonia or nitrite are high. Any way of bringing bacteria in (water or media from another system, pond mud, barnyard soil or starter fish) can bring disease in with it, so weigh it against biosecurity (SRAC 4502). Brand-new, shiny media can be slow to colonise, and some plastic bead media resists bacteria at first (SRAC 4502). Bleach or chlorine left in the system kills the bacteria; a system disinfected between crops has to be cycled again (SRAC 4502). Baking soda: SRAC 4502 and SRAC 452 add sodium bicarbonate to supply alkalinity because they were written for fish-only systems, but SRAC 454 and FAO say never use it in aquaponics, because the sodium harms plants; use a potassium or calcium buffer instead (see pH adjusters). Many fish treatments, including formalin, copper sulfate, potassium permanganate and some antibiotics, inhibit the biofilter (SRAC 452). If you have to cycle with fish, FAO says to stock very lightly (it gives a figure of about 1 kg of fish per cubic metre).
What to buy1, 2
A freshwater test kit that covers ammonia, nitrite and nitrate; FAO says colour-coded drop kits (which usually also test pH, GH and KH) are easy to use, while strips are cheaper and less accurate. For the ammonia source, either fresh fish feed or household ammonia whose label shows nothing but ammonia and water (FAO: no detergents, colourants or other ingredients; SRAC 4502: clear, unscented). Bottled bacteria are optional (see Starter bacteria).
What this page doesn't cover yet
- Ammonia dose disagrees: FAO targets 1 to 2 mg/L and says change water above 3 mg/L; SRAC 4502 and HS1252 dose to 3 to 5 mg/L; SRAC 452's pre-activation medium uses 10 to 20 mg/L. The page gives FAO's figure as the working target because FAO is the only one written for small aquaponic units; a reviewer should confirm.
- Time to cycle differs by source: FAO 3 to 5 weeks (chapter 5) and 3 to 6 weeks (chapter 8), 25 to 40 days to the end point; SRAC 452 at least 1 month; HS1252 3 to 4 weeks before fish.
- The term 'fishless cycling' is the hobby name; the sources describe the method (starting the biofilter before stocking) without that label.
- FAO's fish-in stocking figure appears in the PDF text as '( 1 kg/m3)' with a symbol lost in extraction (probably 'no more than'); check the printed page before quoting it exactly.
- HS1252 is an archival copy: EDIS now returns 'gone' for its publication page. UF/IFAS has no current aquaponics start-up document that we found.
- No source gives Florida-specific cycling times; the Florida note's seasonal remark is our reading of the temperature figures.