How to Add Fish to Your Aquarium

Published October 3, 2026. Built by Jeremy Panasuk.

Quick answer: The Merck Veterinary Manual says a 30-day period is the minimum quarantine for pet fish, and longer may be necessary. Dip water, if you use a dip, should match the tank in pH and temperature. On a cycled 20-gallon tank, this site's calculator calls a first group of 6 small fish comfortable at 30% of capacity. That status is not a reason to skip quarantine or to add the rest of the list the same day.

Quarantine before the display tank

Merck's management chapter recommends quarantine for pet fish. Thirty days is the minimum, and longer periods may be necessary. Quarantined animals are handled only after contact with all other animals is finished for the day. They get their own nets, buckets, and siphons, and they stay separate from the other animals. Disinfectant goes on equipment and in footbaths at the entry to the quarantine area. A history of previous treatment or disease outbreaks should be taken when the fish arrive, and the fish should be examined early in the quarantine period.

The home setup named in that chapter is modest: an inexpensive 10-gallon tank, a sponge filter, a small aeration pump, and a heater. After the fish clear quarantine, disinfect the tank and equipment and store them dry until the next use. During quarantine, nets and siphon hoses are used only on that tank. Before the next purchase, the chapter says the hobbyist can run the previously disinfected sponge filter in an established, disease-free tank so nitrifying bacteria colonize the sponge. That step is specifically to avoid the water-quality problems of a newly set up aquarium. The display-tank side of that problem is how to fix cloudy water in a new tank.

Quarantine does not catch every pathogen. The chapter names mycobacteria, which at the time of that review could not be detected with nonlethal tests, and many viruses. It still says quarantine should be done. Koi are a separate rule on the same page: quarantine at least 30 days at 24°C (75°F) because koi herpesvirus is serious and reportable. Fish that become ill during that quarantine should be tested for it.

Match the water you move them through

This page does not give a bag-floating time. The management chapter does not publish one. What it does say, for dips used on freshwater or marine fish, is that the pH and temperature of the dip water should be the same as the tank water. Freshwater fish are usually exposed to a 30 g/L dip for 0.5–10 minutes, depending on species. If the fish show distress, commonly by rolling onto their side, they should be removed immediately. That dip is a treatment step in the manual, not an acclimation ritual, and the duration depends on the species. A veterinarian sets it. The temperature match is the part that applies every time fish change water: the new water should not be a different temperature from the water they just left.

Fishless cycling of the display tank still comes first. The same chapter says one way to avoid new tank syndrome is to set the tank up with no fish, add ammonia to at least 1–5 mg/L, and add fish only once ammonia and nitrite are both absent. Slowly adding several fish over several months is the other method it names, and it can still produce high ammonia or nitrite if it is not done carefully. The chapter says that practice can be inhumane.

Worked example: the first group in a 20-gallon tank

Points, watts, and turnover on this site are planning estimates for a cycled freshwater tank, not a published stocking standard. The numbers below are the calculator functions. Enter them on the stocking calculator.

20 gallons, normal setup, 6 small fish: 6 points of 20 capacity, 30%, comfortable. Eight small fish on the same tank are 8 points, 40%, still comfortable. Neither result is the number of fish to pour in on the day the tank is filled.

Six small fish are 6 × 1 = 6 points. Capacity is round(20 × 1.0) = 20. Percent is round(6 / 20 × 100) = 30. Status is comfortable because 30 is at or under 80. Eight small fish, a fuller first list some people buy at once, are 8 points, round(8 / 20 × 100) = 40, still comfortable, and still only appropriate after the biofilter is actually converting ammonia and nitrite. The 20-gallon stocking page has longer lists. Those lists are not the first addition.

The quarantine tank Merck names is 10 gallons with a sponge filter, not a hang-on-back sized to a turnover chart. For reference, the same calculator's normal filter function on 10 gallons is 5 turnovers, an ideal 50 gallons per hour, and a derated rating of 60 gallons per hour. A sponge is chosen because the manual names a sponge, and because the flow should stay gentle enough for fish that are already being watched for illness. Heater on that 10-gallon quarantine tank, room 68°F to a target of 78°F: rise of 10°F, 4 watts per gallon, raw load 40 watts, next standard size 50 watts. On the 20-gallon display tank with the same room and target, the raw load is 80 watts and the next standard size is 100 watts. The two-heater note in the function starts at 40 gallons, so neither of these examples asks for two heaters.

Filter on the 20-gallon display tank, normal level: 5 turnovers, ideal 100 gallons per hour, derated rating ceil(100 × 1.2 / 5) × 5 = 120 gallons per hour. See what size filter a 20-gallon tank needs for the rest of that page's filter discussion.

A common mistake

Putting the store bag straight into the display tank, and putting the whole shopping list in on the same afternoon. Thirty days is the minimum quarantine in the manual, not a suggestion for when you have a spare tank. Six small fish at 30% is a first group on a cycled tank. It is not a smaller version of skipping the quarantine.

Limits, and when to call a veterinarian

Fish that get sick in quarantine need a diagnosis, not a guess. Koi herpesvirus is reportable; the manual says fish that become ill during the koi quarantine should be tested for it. Quarantine will not reveal every infection, which is why a fish that crashes after you followed the 30 days still belongs with an aquatic veterinarian. Drug doses are not on this page. Feeding once the fish are in is covered in how much to feed tropical fish, and the starter sequence is in how to keep pet fish alive.

Shop an aquarium sponge filter on Amazon

Affiliate Affiliate disclosure: as an Amazon Associate, we earn from qualifying purchases — at no cost to you. A sponge filter is listed because the Merck chapter names one for the 10-gallon quarantine tank. This site has not tested a brand.

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

Sources

Petty BD, Francis-Floyd R, Yanong RPE. Management of Aquarium Fish. Merck Veterinary Manual. Full review May 2022. Last updated June 2026. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/management-of-aquarium-fish. Accessed October 3, 2026.