How to Add New Fish to an Aquarium: Acclimatisation and Quarantine
Most new fish deaths happen in the first 72 hours and almost all of them are preventable. Here is the acclimatisation and quarantine routine that gets fish through the transition.
A fish arriving at your home has been netted, bagged, transported, and moved between water that may differ from yours by several degrees, a full point of pH, and a large gap in hardness. It has also not eaten. That combination, not disease, is what kills most new fish in the first three days.
Before the Fish Arrive
- Confirm the tank is fully cycled: ammonia 0, nitrite 0, nitrate under 40 ppm.
- Check the temperature is stable and correct for the species.
- Do a 20% water change a day or two before, not on the same day.
- Have hiding places ready: plants, caves, driftwood. New fish need somewhere to feel safe.
- Dim or switch off the tank light. Bright light on arrival adds stress.
- If you plan to quarantine, have that tank running with cycled media already.
Method 1: Float and Mix (Standard, 45 Minutes)
Suitable for hardy species: guppies, mollies, platies, tetras, barbs, goldfish, most cichlids.
- Turn off the aquarium light and dim the room.
- Float the sealed bag on the tank surface for 15-20 minutes so temperatures equalise.
- Open the bag and roll the top down so it floats on its own.
- Add about half a cup of tank water to the bag. Wait 10 minutes.
- Repeat three or four times over 30-40 minutes, until the bag holds roughly twice its original water.
- Net the fish out and place them in the tank. Discard the bag water down the sink, never into the aquarium.
- Leave the lights off for a few hours and do not feed until the next day.
⚠️Never pour the shop or shipping bag water into your aquarium. It carries concentrated ammonia and whatever pathogens were in the seller tank. Always net the fish across.
Method 2: Drip Acclimatisation (Sensitive Species, 1-2 Hours)
Use for discus, wild caught species, shrimp, marine livestock, and any fish that has been in transit for more than a day.
- Pour the bag contents (fish plus water) into a clean bucket or large container placed below tank level.
- Start a siphon from the tank using airline tubing, and tie a loose knot in the tubing or use an airline control valve to slow it to 2-4 drips per second.
- Let it drip until the water volume in the bucket has roughly tripled. This usually takes 45-90 minutes.
- Discard half the bucket and drip again for another 20 minutes if the source water was very different.
- Net the fish into the tank. Do not add the bucket water.
Quarantine: Worth the Trouble
A quarantine tank is a bare 1.5 to 2 foot tank with a sponge filter, a heater, and a couple of hiding places. New fish live there for 2 to 4 weeks before joining the main tank. It costs around ₹2,000-₹4,000 to set up and it prevents the single most expensive mistake in fishkeeping: introducing ich, velvet or columnaris into an established display tank and losing everything in it.
| Week | What to do | What to watch for |
|---|---|---|
| Week 1 | Observe only, feed lightly, keep lights low | White spots, clamped fins, refusal to eat, rapid breathing |
| Week 2 | Normal feeding, daily observation, small water changes | Fin rot, fungus, unusual swimming, weight loss |
| Week 3-4 | Treat anything that appeared; otherwise just observe | Any symptom resets the clock |
| End | Acclimatise into the main tank as if newly bought | Add during a low activity period, lights off |
Keep a spare sponge filter running permanently in your main tank. When you need a quarantine tank, move that sponge across and the quarantine tank is instantly cycled. This one trick makes quarantine practical rather than theoretical.
How Many Fish to Add at Once
| Tank age and size | Safe to add at once | Then wait |
|---|---|---|
| Newly cycled 1.5-2 ft | 2-3 small fish | 2 weeks |
| Newly cycled 3 ft | 4-6 small fish | 2 weeks |
| Mature 2 ft tank | 3-5 small fish | 1-2 weeks |
| Mature 3-4 ft tank | 6-8 small fish or 1-2 large | 1-2 weeks |
| Any tank, large messy fish | 1 fish | 3-4 weeks |
Adding a whole shopping bag of fish to a young tank overloads the bacteria colony and causes an ammonia spike. The bacteria population grows to match the waste load, and that takes days to weeks. Test ammonia 48 hours after adding fish: if it is above zero, do a water change and hold off on further additions.
Introducing Fish to an Aggressive Tank
- Rearrange the decor before adding new fish. This breaks up established territories and gives the newcomer a fair start.
- Add new fish in the evening with the lights off.
- Add schooling fish as a full group, never one at a time.
- For cichlids, add the new fish while the existing ones are distracted by food, or use a breeder box for a day so they can see each other without contact.
- Watch for the first hour. Chasing that does not settle within a day means the pairing will not work.
First Week Aftercare
- Do not feed on arrival day. A stressed fish will not eat and the food will rot.
- From day two, feed small amounts and check that the new fish is actually getting some.
- Test ammonia and nitrite on day 2 and day 5.
- Keep lighting shorter than usual for the first few days.
- Do not do a large water change in the first 48 hours unless ammonia demands it.
- Expect hiding. A fish that hides for two days and then starts exploring is normal.