Aquarium Setup and Maintenance: Cycling, Water Tests and Routine

This page is for anyone setting up a freshwater aquarium or taking over one that keeps losing fish. It covers the order of the nitrogen cycle, how to cycle without fish, what numbers your test kits should show, and the weekly and monthly jobs that keep those numbers there.

Water parameters to aim for

Parameter	Target	Why it matters
Ammonia (total, TAN)	0 mg/L (RSPCA)	Toxic; the un-ionized form is about 100 times more toxic than ammonium, and its share rises with pH and temperature (UF/IFAS)
Nitrite	0 mg/L (RSPCA)	Toxic to fish at levels as low as 0.10 mg/L (UF/IFAS)
Nitrate	Keep low with water changes; no single safe figure is agreed	Once treated as harmless up to 200 mg/L, but newer research suggests it is more harmful than thought; closed tanks can exceed 250 mg/L (UF/IFAS)
pH	Species-specific; FishBase lists 6.0–8.0 for both goldfish and freshwater angelfish (FishBase)	Low alkalinity lets pH crash suddenly (Merck)
Alkalinity	At least 100 mg/L as CaCO₃; 50 mg/L or less stalls the biofilter (Merck)	Nitrifying bacteria use up bicarbonate (UF/IFAS)
Temperature, cold-water	Goldfish 50–70°F (10–21°C); a heater usually isn’t needed (RSPCA)
Temperature, tropical	Set for the species, e.g. angelfish 75–86°F (24–30°C) (FishBase); always heated (RSPCA)	Heaters and thermostats fail, so check a digital thermometer daily (RSPCA)
Chlorine/chloramine	Below 0.1 mg/L total chlorine (City of Sioux Falls)	Damages gills and blood oxygen transport

Cold-water and tropical fish shouldn’t share a tank: the temperatures above barely overlap. See the goldfish page for cold-water setups and the betta, angelfish and oscar pages for heated tanks.

Treat every refill: chlorine and chloramine

Tap water is disinfected with free chlorine or with chloramine (chlorine bound to ammonia), and both harm fish and the filter bacteria (RSPCA; City of Sioux Falls). Add a water conditioner to every bucket of new water, including top-ups. The two disinfectants behave differently:

• Chlorine leaves water left to stand for 24 hours (RSPCA).
• Chloramine does not. It can take weeks to disappear, and boiling and reverse osmosis don’t remove it (City of Sioux Falls). The RSPCA’s “leave it standing” option only works if your supplier uses plain chlorine.
• When a conditioner breaks chloramine apart, ammonia is released, so choose a product labelled for chloramine and ammonia, or use granular activated carbon (City of Sioux Falls).

Match the new water’s temperature to the tank before adding it, or the change can shock the fish (RSPCA).

The nitrogen cycle and fishless cycling

Fish excrete ammonia. Ammonia-oxidizing bacteria (such as Nitrosomonas) turn it into nitrite, and nitrite-oxidizing bacteria (such as Nitrospira and Nitrobacter) turn nitrite into nitrate (UF/IFAS). Both groups need oxygen and alkalinity and live mainly in the filter. A new biofilter takes about six to eight weeks to establish (UF/IFAS; Merck). VCA gives 4–6 weeks of cycling and the RSPCA says “at least two weeks”, so go by test results rather than the calendar.

Fishless cycling, in order (Merck; RSPCA):

1. Set up the tank completely: filter, heater if needed, substrate, conditioned water.
2. Add an ammonia source to reach 1–5 mg/L (Merck).
3. Test ammonia, nitrite and nitrate every other day (RSPCA).
4. Expect ammonia to fall as nitrite rises, then nitrite to fall as nitrate appears.
5. Add fish only once ammonia and nitrite both read zero after their spike (RSPCA; Merck). Stock a few at a time.

Bacterial starters. The RSPCA suggests seeding with a starter product or with filter media from a mature, disease-free tank. Bottled products may shorten cycling, but how well they work varies, so they don’t replace testing. Seeded media from a healthy tank is the better-supported option: Merck recommends running a sponge filter in an established tank before you need it. Adding fish one by one to cycle the tank can push ammonia and nitrite to toxic levels, and Merck calls the practice potentially inhumane.

Weekly and monthly maintenance checklist

When	Task
Daily	Read the thermometer; check that every fish is eating and the filter is running (RSPCA)
Weekly	Change about 30% of the water to dilute nitrate (RSPCA); vacuum gravel in sections; conditioner in all new water
Every 2 weeks	Test ammonia and nitrite; these kits are “essential, not optional” (RSPCA)
Monthly, or sooner if flow drops	Swish the filter sponge in removed tank water, never under the tap; clean or replace media in stages, not all at once (RSPCA)
Any reading above 0	Change half the water daily and retest until ammonia and nitrite return to zero (RSPCA)

VCA’s minimum is a partial change every 2–4 weeks. The RSPCA’s weekly 30% suits most community tanks better, and heavy feeders need more. Merck’s “old tank syndrome” shows why: in tanks with large carnivores such as oscars, small, infrequent changes and topping up only for evaporation let alkalinity run out until pH drops, sometimes fast enough to kill fish. Correct an old tank with small daily changes, not one big one, because a sudden pH rise turns the accumulated ammonia toxic (Merck).

Mistakes that kill fish

• Adding a full stock to a two-week-old tank. New tank syndrome usually strikes within the first 6 weeks: fish seem fine, then turn lethargic, stop eating and die from ammonia or nitrite (Merck).
• Rinsing filter media under the tap. Chlorine kills the bacteria, and the cycle starts again (RSPCA).
• Assuming plants guarantee oxygen. Plants and algae produce oxygen only in light. At night they respire and use oxygen, so a heavily planted tank with poor surface movement can run short before dawn. Fish are distressed at 2–4 mg/L dissolved oxygen and die below 2 mg/L, and warm water holds less: 7.4 mg/L at 90°F against 11.9 mg/L at 45°F (UF/IFAS).
• Tall, narrow tanks. Oxygen enters at the surface, so wide tanks are better (RSPCA).
• Skipping quarantine. Merck’s minimum is 30 days in a separate tank with its own nets and siphon.

Avoid: untreated tap water, free-chlorine-only test kits on chloraminated supplies, replacing all filter media at once, and copper treatments in the display tank (copper suppresses nitrifying bacteria for weeks to months, per Merck). For fish health problems, see small pet veterinary care and quarantine protocols.