AquariumStocking

How to Keep Pet Fish Alive

Published October 3, 2026. Built by Jeremy Panasuk.

Quick answer: Cycle with no fish until ammonia and nitrite are gone, test those two, then add a small first group. On one 20-gallon starter, this site's calculator puts 6 small fish at 30% of capacity, a 68°F room headed for 78°F on a 100-watt heater, and a normal filter at 120 gallons per hour. The water-change example below uses the calculator's sample inputs, not a test from your tank.

The deeper pages are cloudy water in a new tank, how to add fish, how much to feed, and how many fish in a 20-gallon tank. This page is the order of operations for one setup, not a second copy of those articles.

1. No fish until ammonia and nitrite are gone

The Merck Veterinary Manual's management chapter calls new tank syndrome a problem that usually shows up in the first 6 weeks: fish turn lethargic, stop eating, and frequently die, and ammonia, nitrite, or both are often toxic. A tropical biofilter, in that same chapter, can take up to 8 weeks to establish. The environmental-diseases chapter uses different wording on purpose: un-ionized ammonia rises in the first 2–3 weeks and/or nitrite rises after that, and a new biofilter usually takes about 6 weeks to become completely established. Do not collapse "usually within 6 weeks," "up to 8 weeks," "the first 2–3 weeks," and "about 6 weeks" into one countdown.

Fishless cycling, from the management chapter, is the avoidance method: set the tank up with no fish, add ammonia to at least 1–5 mg/L, test ammonia, nitrite, and nitrate, and add fish only when ammonia and nitrite are both absent. SRAC Publication No. 452, a March 1999 aquaculture sheet about recirculating systems rather than a home tank, says activation that can handle normal feeding rates takes at least 1 month, and feeding stops at the first sign of high ammonia. Use it as a warning about food during startup, not as a substitute calendar.

2. Test, and keep the thresholds apart

Most kits read total ammonia nitrogen. Under most circumstances freshwater fish tolerate TAN under 1 mg/L. In most home aquariums a TAN of 1.0 is not a huge concern unless pH exceeds 8.5, and you still calculate the un-ionized fraction. Un-ionized ammonia under 0.05 mg/L is not considered harmful, with species and situation differences. A calculated 0.05 mg/L is already enough for gill damage and is considered harmful. Un-ionized ammonia of 2 mg/L is lethal for many fish. The new-tank table row is another line: un-ionized ammonia above 1 mg/L and pH above 8. Nitrite-nitrogen readings convert to nitrite by multiplying by 3.3. The table flags nitrite above 0.1 mg/L when chloride is low. Nitrate on a brand-new tank is usually 0 mg/L, so a pretty nitrate number does not clear the tank.

3. Four calculator results for this one 20-gallon tank

Points, watts, turnover, and water-change percent on this site are planning estimates for a cycled freshwater tank, not a published standard. Every input is written out. The same four functions are on the calculator.

Stocking. 20 gallons, normal style, 6 small fish, nothing else: 6 points, capacity round(20 × 1.0) = 20, percent round(6 / 20 × 100) = 30, comfortable because 30 is at or under 80. That is the first group, not a full community.

Eight small fish would be 8 points and 40%, still comfortable, and still not a day-one load. The management chapter says adding several fish over several months can raise ammonia or nitrite if you are careless, and that the practice can be inhumane. Thirty percent is headroom after the cycle, plus quarantine of at least 30 days before those fish join an established tank. Adult size is what you enter. A juvenile that will be a large fish is not a "small" point.

Heater. 20 gallons, room 68°F, target 78°F. The rise is 10°F. Watts per gallon are 4, because the rise is above 9 and at most 15. Raw watts are 80. The next standard size at or above 80 is 100 watts. The two-heater note starts at 40 gallons, so this tank is one heater.
Filter. 20 gallons, normal level: 5 turnovers an hour, ideal flow 100 gallons per hour, derated rating ceil(100 × 1.2 / 5) × 5 = 120 gallons per hour. The rating assumes the bacteria already live in the media. A new filter with a new tank is still the fishless cycle above.
Water change. Example inputs only, not a reading from a tank: nitrate rises 20 ppm in a week, you do not want the plateau above 40 ppm, and the tap water is 5 ppm. Fraction = 20 / (40 − 5) = 20/35. Rounded up to a 5% step, ceil((20/35) × 20) × 5 = 60%. Plateau at that change is round(5 + 20 / 0.60) = 38 ppm.

If your own weekly tests show a different rise or a different tap nitrate, enter those. Do not reuse 20, 40, and 5 as if they were measured in this 20-gallon starter. Feeding is what pushes that rise around: UF/IFAS VM114 says one feeding a day and 0.5–1.0% of body weight is enough for maintenance of most pet animals, and that over-feeding is the usual way pet owners wreck water quality. There is no sourced body weight on this page, so there is no gram dose.

A common mistake

Buying the heater, the filter, the food, and every fish on the same Saturday, then putting the fish in that night. The 100 watts and the 120 gallons per hour do not make the tank cycled. Six small fish at 30% is the first addition after ammonia and nitrite read absent, and after quarantine if the tank already has animals in it.

Limits, and when to call a veterinarian

This is a planning list. It is not a medical exam. Fish that are lethargic, off food, spinning, or dying need the sick-fish page and an aquatic veterinarian, especially inside the first 6 weeks, when new tank syndrome is the usual water failure. Alkalinity at or below 50 mg CaCO3/L inhibits the biofilter; the management chapter says to buffer so alkalinity is at least 100 mg CaCO3/L. If you are keeping koi rather than tropical community fish, the 20-gallon heater result does not transfer to a pond. Use the 500-gallon koi page, which does not run an aquarium heater against pond water.

Shop an aquarium liquid test kit on Amazon

Affiliate Affiliate disclosure: as an Amazon Associate, we earn from qualifying purchases — at no cost to you. A liquid test kit is listed because the checklist depends on ammonia and nitrite readings. This site has not reviewed a brand.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

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