How to Acclimate New Fish (and When Slow Acclimation Makes Things Worse)

Most fish that die in their first week did not die of the tank. They died of the twenty minutes between the car and the water.

Acclimation gets taught as one rule — float the bag, then drip for an hour — and that rule is right about half the time and actively harmful the other half. Which half you are in depends on something almost nobody checks: how long the fish has been sitting in that bag.

What is happening inside the bag

The moment the store ties the knot, three things start moving.

The fish excretes ammonia. In a sealed bag with no filter, it accumulates with nowhere to go. Over a two-hour drive a small bag can reach 1 ppm or more.

At the same time the fish exhales CO2, which dissolves and forms carbonic acid, and the pH of the bag water drops. A bag that left the store at 7.4 can easily read 6.8 by the time you get home. Longer trips, from a mail-order shipment, can drop further.

Here is the part that matters. Ammonia exists in water in two forms: ammonium (NH4+), which is relatively harmless, and free ammonia (NH3), which burns gills. Which one you get is governed by pH. Below about pH 7, nearly all of it sits as harmless ammonium. Above pH 7.5, a meaningful fraction converts to the toxic form, and the fraction roughly multiplies for each additional pH point.

So the fish has been floating in an acidic bag full of ammonium, and it has been fine. Then you open the bag.

The trap

Opening the bag lets the accumulated CO2 gas off. The pH starts climbing back toward normal within minutes. Now add tank water — usually pH 7.4 or higher — a splash at a time, over an hour.

You have just converted a bag of harmless ammonium into a bag of ammonia burn, and you are holding the fish in it while you drip. The slow, careful, textbook acclimation is the thing doing the damage.

I have watched this happen. A bag of cardinal tetras from a three-hour shipping run, dripped for ninety minutes because I was being conscientious, and by the time they went in they were flashing and gasping. Two didn't make the week.

The rule I actually use

Short trip home (under about an hour): drip acclimate. Ammonia has not had time to build to anything meaningful, so the slow route is safe and the fish gets a gentle transition on temperature and hardness.

Long trip, or any shipped fish: get them out fast. Float the sealed bag 15 minutes to match temperature, then net the fish straight into the tank. Do not drip. Do not add tank water to the bag. Ten to fifteen minutes total, start to finish. A brief hardness shock is far less dangerous than forty minutes in rising-pH ammonia water.

That second one feels wrong the first time you do it. Do it anyway. Every serious shipper of fish, from wholesalers to the people mailing you rare plecos, gives the same instruction, and it is for exactly this reason.

Drip acclimation, done properly

For the short-trip case, this is the method:

Pour the bag — fish and water — into a clean bucket or a plastic tub. Never a bucket that has met soap. Tilt it so the fish has depth to sit in. Note roughly how much water you started with, because the target is doubling it.

Run a length of airline tubing from the tank down to the bucket, start a siphon, and tie a loose knot in the tubing to throttle it. You want 2 to 4 drops per second. A cheap plastic control valve does the same job for about two dollars and is less annoying.

Let it run until the volume has doubled, then stop. That is usually 30 to 45 minutes and it is enough — you are not aiming for identical water, only for a gradual approach. If the bucket is filling faster than that, close the knot.

Then net the fish into the tank and throw the bucket water away. Not down the tank. That water is store water plus transport ammonia, and there is no reason for any of it to enter your system.

Lights off for the rest of the day, and no food until tomorrow. A fish that has just moved house is not hungry, and uneaten flake is how a new tank gets its first ammonia spike.

Inverts play by different rules

Shrimp and snails need the slow version regardless of trip length, and they need it slower — two hours of dripping for neocaridina is normal. They tolerate ammonia about as poorly as fish do, but the bag chemistry is gentler because their waste output is tiny, so the trap above mostly doesn't apply.

For shrimp the parameter that matters is not pH but TDS — total dissolved solids — and a jump of more than about 50 ppm is what triggers a failed molt a few days later. That death is delayed enough that most people never connect it to the acclimation. A cheap TDS pen is worth more to a shrimp keeper than a full liquid test kit.

Saltwater is the same story with salinity. Match specific gravity to within 0.001 and drip slowly.

Quarantine, briefly

The right answer is a bare 10-gallon with a sponge filter and a heater, four weeks, before anything joins the display tank. Ich, velvet, and internal parasites all show up inside that window, and treating one small tank is far easier than treating an aquascape full of shrimp and plants that hate copper.

Most hobbyists skip it. I skipped it for years. What finally converted me was losing a settled community to a single new fish carrying velvet — a hundred-dollar mistake avoidable by a twenty-dollar tank.

If you will not run a quarantine tank, at least stagger your purchases so a disease outbreak has a smaller starting population, and never buy from a store tank with a dead fish in it. Not "avoid that tank" — avoid that store, that day. Store systems are usually plumbed together.

Before you buy anything

Acclimation only matters if the fish had a place to go. Check the stocking first: run the tank through Fish That Fit so you know the bioload works and nobody in the tank eats anybody, and make sure your water parameters are stable and your cycle is finished. A perfectly acclimated fish going into an uncycled tank is still a dead fish, just on a slower schedule.