Acclimate the fish to your tank water before releasing them

Fish cannot go straight from the bag they came in into your tank. The temperature, pH, and salt content (if saltwater) of the bag water are almost always different from your tank water, and a sudden change can shock or kill the fish. Acclimation is the process of gradually mixing the two waters so the fish adjust over 20 to 30 minutes instead of seconds.

When you bring the fish home, float the sealed bag in your tank for 15 minutes so the water inside reaches the same temperature as the tank. Then open the bag and begin adding small amounts of tank water to it — about a quarter cup every few minutes — for the next 15 to 20 minutes. Once the bag water and tank water are roughly the same, use a net to move the fish into the tank and discard the bag water. Do not pour the bag water into the tank, because it may contain waste or disease from the store.

Key Takeaways

  • Float the sealed bag in your tank for 15 minutes to match water temperature, then slowly add tank water to the bag over 15 to 20 minutes before netting the fish in.
  • Never pour the bag water into your tank; use a net to transfer only the fish.
  • Add fish in small groups over several days or weeks rather than all at once, so the filter can handle the waste load.
  • Watch for signs of stress in the first few hours: gasping at the surface, hiding, or darting erratically are signs the acclimation was too fast or the water conditions are wrong.
  • Keep the tank lights off for a few hours after adding fish so they have time to explore and hide without the stress of bright light.

Add fish gradually, not all at once

Your tank's filter and beneficial bacteria can only handle a certain amount of fish waste. If you add too many fish in one day, ammonia and nitrite will spike and poison the water, killing the fish you just added. The rule of thumb is one inch of fish per gallon of water — so a 20-gallon tank can hold about 20 inches of fish total — but this is a starting point, not a hard limit.

Space out your additions over at least a week, and longer if you are stocking a new tank. Add a small group of hardy fish first (like tetras or danios), wait three to five days, then add the next group. This gives the bacteria time to multiply and handle the extra waste. If you test your water and ammonia or nitrite is above zero, wait longer before adding more fish.

Check water conditions before and after adding fish

Before you bring any fish home, test your tank water for ammonia, nitrite, nitrate, pH, and temperature. Ammonia and nitrite should both be zero; if either is above zero, your tank is not ready and the fish will die. Nitrate can be up to 20 parts per million (ppm) in an established tank, but zero is better.

After you add fish, test again after 24 hours and then every few days for the first two weeks. If ammonia or nitrite rises above 0.5 ppm, do a 25 percent water change when ready and wait several more days before adding any more fish. A water test kit costs $20 to $35 and pays for itself by preventing dead fish and wasted money.

Use a net to catch and move fish safely

A fish net with a soft mesh bag (not a hard net) reduces stress and injury. Wet the net before use so the fish does not stick to it. Move slowly and deliberately — chasing the fish around the tank exhausts it and damages its fins. If the fish is in a bag, tilt the bag slightly and guide the fish into the net rather than scooping.

Once the fish is in the net, hold it over the tank and gently lower it into the water. Let the fish swim out on its own rather than shaking it loose. If the fish is injured or bleeding, it is more vulnerable to infection; in that case, add a small amount of aquarium salt (not table salt) to the tank according to the product instructions, or move the fish to a separate quarantine tank for a few days.

Keep the tank dark and quiet for the first few hours

New fish are stressed and disoriented. Bright light and sudden movement make it worse. Turn off the tank lights for at least two to four hours after adding fish, and keep noise and vibration to a minimum. The fish will spend this time exploring hiding spots and adjusting to the new space.

After the lights come back on, watch the fish for the first hour. Normal behavior is exploring, eating if food is offered, and hiding occasionally. Signs of serious stress are gasping at the surface, staying motionless at the bottom, or darting frantically in one direction. If you see these signs, check the water temperature and ammonia level when ready. A temperature drop of even a few degrees or a spike in ammonia can cause this behavior.

Do not feed fish for the first 24 hours

A new fish's digestive system is stressed by the move, and uneaten food will foul the water and spike ammonia. Wait at least 24 hours before offering food, and then feed only a small pinch. The fish can go several days without food and will not starve.

Once the fish is eating normally and the water tests stable, return to your regular feeding schedule. Overfeeding is one of the easiest ways to crash water quality in a new tank, so err on the side of feeding less rather than more during the first week.

Quarantine new fish if you have an existing tank with other fish

If your main tank already has fish, a new fish may carry parasites or disease that will infect the others. The safest approach is to keep new fish in a separate quarantine tank — even a 5 or 10-gallon tank with a filter and heater — for two to four weeks before moving them to the main tank. Watch for spots, torn fins, or unusual behavior. If the new fish stays healthy and the quarantine tank water stays stable, move it to the main tank using the same acclimation process.

If you do not have a quarantine tank and the new fish looks healthy, add it to the main tank but watch all the fish closely for the next week. Any sign of disease (white spots, torn fins, lethargy) means isolating the sick fish when ready or treating the whole tank with medication.

Frequently Asked Questions

How long should I float the bag before adding fish?

Float the sealed bag for 15 minutes to match the water temperature. Then spend another 15 to 20 minutes slowly adding tank water to the bag. The entire process takes about 30 to 40 minutes. Rushing this step is the most common reason new fish die in the first few hours.

Can I add multiple fish at the same time?

Yes, you can add several fish in one session as long as the total does not exceed your tank's capacity and you space out future additions. A small group of the same species (like five tetras) is often less stressful than adding one fish alone, because they have companions. But do not add a full tank of fish in one day.

What does it mean if a new fish hides all day?

Some hiding is normal for the first day or two. But if the fish hides for more than 48 hours and does not eat, check the water temperature, ammonia, and nitrite. A hidden fish that is not eating usually means the water conditions are wrong or the fish is sick.

Do I need to turn off the filter when I add fish?

No, keep the filter running. The filter helps remove waste and keeps the water oxygenated. A strong current can stress a new fish, so if your filter outlet creates a visible current, point it at the tank wall instead of the center for the first few days.

What if the fish dies shortly after I add it?

Test the water when ready for ammonia, nitrite, and temperature. If ammonia or nitrite is above zero, the tank was not ready. If the water is fine, the fish may have been sick at the store or injured during transport. Either way, do not add more fish until the water tests stable for several days.