You’ve cycled the tank, planned the stocking, and just brought fish home from the aquatic shop. Now the fish are in a sealed bag full of shop water, and your tank has different water. Add them wrong and they die within days from parameter shock — even into a perfectly-cycled tank.
This guide covers the three acclimation methods, when to use each, and the specific parameter-shock scenarios most likely to kill new fish in UK community tanks.
This article contains affiliate links. If you buy through one of them, FishForever earns a small commission at no cost to you. We only recommend products we’d use ourselves. See our Affiliate Disclosure.
- Acclimate hardy fish with the float method (30 minutes); use the slower drip method (60–90 minutes) for sensitive species and shrimp.
- Never pour shop bag water into your tank — net the fish out instead.
- Turn tank lights off and do not feed on arrival day to reduce stress.
- Ideally quarantine new fish for 4–6 weeks before adding them to the main community.
Why acclimation matters
Shop water and your tank water are almost never identical. Differences in:
- Temperature (shop tank may be warmer or cooler than your bag by the time you get home)
- pH (typically 0.2–1.0 units different)
- Hardness (particularly if you’re in a soft-water area buying from a hard-water shop)
- Nitrate levels (shop tanks often have much higher nitrate than home tanks)
- Dissolved oxygen (bag water may be low)
Adding fish directly (“plop and drop”) into significantly different water causes osmotic shock — fish gill cells rupture as they try to regulate salt/water balance against a sudden change. Symptoms: gasping, loss of colour, hanging at the surface or bottom. Death within 24–72 hours.
The three methods
1. Float method (basic, quick — 30 minutes)
Best for: hardy species (danios, most livebearers, tetras of established shops), short bag transit (under 30 minutes home), similar-parameter shop and home water.
How:
- Turn tank lights OFF. Reduces stress on the new fish.
- Float the sealed bag in the tank for 15–20 minutes — matches temperature.
- Open the bag and roll the top down to form a floating collar.
- Every 5 minutes, add half a cup (~100 ml) of tank water to the bag. Do this 4–5 times over 20 minutes.
- Use a net to lift the fish OUT of the bag into the tank. Do NOT pour bag water into the tank — shop water can carry disease/parasites.
- Discard the bag water.
Total time: 30 minutes.
2. Drip method (best for sensitive species — 60–90 minutes)
Best for: otocinclus catfish, cardinal tetras, cherry shrimp, amano shrimp, corydoras, ram cichlids, discus, other sensitive species.
Kit needed:
- Bucket (dedicated to aquarium use)
- 1m of airline tubing
- Airline valve/knot for flow control
How:
- Turn tank lights OFF.
- Empty the bag (fish + water) into a clean bucket. The bucket should be BELOW the tank level.
- Set up an airline siphon from the tank to the bucket. Suck the airline briefly to start the flow.
- Tie the airline in a loose knot to slow drip to ~1 drop per second. Alternatively use an airline valve.
- Let water drip into the bucket for 60–90 minutes. The bucket volume should about double or triple.
- Once acclimated: net the fish OUT of the bucket into the tank. Discard the bucket water.
Total time: 90 minutes. Much safer for sensitive species than the float method.
3. Plop-and-drop (only for perfect-match parameters)
Best for: Wild-caught blackwater fish being moved from breeder to your tank where water parameters are IDENTICAL. Rare in UK community-tank contexts.
How: Net directly from shop bag into tank. No temperature matching, no water mixing.
When it makes sense: almost never in casual UK aquarium fishkeeping. Included for completeness.
Which method for which situation
| Species | Recommended method |
|---|---|
| Hardy fish (danios, hardy livebearers, common tetras) | Float |
| Bristlenose plec, corydoras | Float or drip |
| Neon/cardinal tetras | Drip |
| Cherry / amano shrimp | Drip |
| Otocinclus | Drip |
| Kuhli loaches | Drip |
| Discus, dwarf cichlids (rams, apistogramma) | Drip |
| Angelfish | Float for juveniles; drip for adults |
| Marine fish | Drip |
When in doubt, use the drip method. It’s slower but never causes harm.
What NOT to do
- Pour bag water into the tank. Introduces shop diseases, parasites, medication residues.
- Skip temperature matching. Even hardy fish shocked by 5°C+ differences.
- Acclimate with tank lights on. Bright lights add stress at exactly the worst moment.
- Feed on day of arrival. New fish are stressed; won’t eat; food fouls water.
- Add multiple new species in one visit. Overwhelms your bacterial colony and mixes multiple potential disease sources.
- Skip acclimation “because it’s just guppies.” Even hardy fish die from parameter shock.
Special cases
Bag opened at home and fish is gasping
Not a shock issue — bag was low-oxygen from long transit. Get fish out of bag quickly:
- Skip drip; do abbreviated float (10 minutes for temperature only)
- Net fish out
- Add to tank
- Air stone running for extra oxygen
Bag arrived cool from transit
Fish are dormant. Warm the bag slowly by floating it in the tank for 30 minutes before beginning acclimation. Do NOT dump directly into warm tank water.
Shrimp specifically
Shrimp are particularly sensitive to sudden hardness changes. For cherry and amano shrimp, use the drip method extended to 90–120 minutes. Also test the bag water hardness with an API GH/KH kit — if bag water is dramatically softer than your tank, drip for longer or accept higher losses.
Newly-arrived shipment (bought online)
Fish shipped overnight often arrive stressed. Standard drip method + reduced feeding for 2 days + minimal disturbance for the first week gives best survival.
Quarantine — the next step
Ideally, new fish should go into a quarantine tank for 4–6 weeks before joining your main community. This isolates any hidden diseases (ich, velvet, parasites) so you can treat them without exposing the main tank.
Basic quarantine tank:
- 30–40L bare-bottom tank
- Sponge filter (seeded from main tank’s mature filter for instant bio-cycling)
- Heater
- A few plastic hiding places (easy to disinfect)
- No substrate (easier to remove waste)
Set up quarantine BEFORE buying fish. It’s the single biggest thing separating hobbyists whose fish die from those whose fish don’t.
Most beginners skip quarantine “just this once.” Then when ich crashes the entire community tank, they wish they hadn’t.
Bottom line
For hardy species from a good shop: float method, 30 minutes, net in.
For sensitive species (shrimp, otos, cardinals, dwarf cichlids): drip method, 60–90 minutes, net in.
Never dump bag water into the main tank. Never feed on arrival day. Ideally, use a quarantine tank for 4–6 weeks before adding to community.
Following these three rules turns “new fish mortality” from a routine annoyance into an unusual event.
Frequently asked questions
How long should I acclimate new fish?
About 30 minutes using the float method for hardy fish, or 60–90 minutes using the drip method for sensitive species like shrimp, otocinclus and cardinal tetras.
Should I add the bag water to my tank?
No. Net the fish out and discard the shop water, which can carry disease, parasites and medication residues into your tank.
What is the drip acclimation method?
You place the fish and bag water in a bucket, then siphon tank water in at about one drop per second for 60–90 minutes, letting the fish adjust gradually before netting them into the tank.
Related: how to set up a fish tank, how to cycle a fish tank, best aquarium fish for UK beginners, ich treatment.
— Jaqline Smalling, FishForever
Leave a Reply