How to Freshwater Dip Fish

Ok, calm down.  This isn’t nearly the rocket science procedure everyone makes it sound like.  Yes, I was terrified the first time I tried it, but after dipping dozens of fish with no losses (due to the freshwater dip), I’m a believer.  So why freshwater dip fish?  A short dip can actually dislodge parasites like black turbellarian worms, Amyloodinium (marine velvet), Uronema, flukes, and other nasties (Noga, 2010).  No, it doesn’t work on marine ich (Cryptocaryon irritans), but still, it’s a great start to having healthy fish.

Injured & Infected Purple Tang

Injured & Infected Purple Tang

There are several ways to go about a freshwater dip, but the instructions posted here are how I prefer to do it.  I add methylene blue to the dip, but you can skip this if desired.  Methylene blue has a lot of benefits with little risk.  For instance, it helps with nitrite poisoning (which is possible if the fish was recently shipped) (Kroupova, 2005).  It is also an antidote for cyanide poisoning (Egekeze, 1980), which unfortunately, still occurs in the hobby.  In fact, I’ve found articles going back prior to 1933 discussing methylene blue as an antidote for cyanide poisoning.  It’s nothing new.

Fish in Freshwater Dip

Fish in Freshwater Dip with Methylene Blue

However, I must highly caution you – don’t try this first on a rare $300+ fish you just picked up.  Try it on inexpensive fish, then move on to more expensive fish as you get more experienced.

Supplies needed:

  • Bucket (I use a 1 gallon bucket, as shown in the photos)
  • Fish net (two nets may help)
  • Thermometer
  • pH probe (calibrated)
  • Syringe or other measuring device (in milliliters)
  • Air pump and air stone (or a powerhead)
  • Methylene Blue
  • Tap Water
  • Dechlorinator

Methylene Blue and Dechlorinator

Methylene Blue and Dechlorinator


1.  Make sure all your supplies are clean and in good working condition.

2.  Acclimate fish properly if they are new to your home.

3.  Fill the bucket with tap water, about 3° Fahrenheit warmer than the tank.  (The water will cool.)

4.  Add the dechlorinator as recommended by the manufacturer’s instructions.  Don’t miss this step!!

5.  Add the airstone and turn the pump on.

6.  Match the pH of the tap water to the tank water.  Tap water is closer than freshwater, which is why I use it.

7.  Add the methylene blue according to manufacturer’s instructions for a dip (I use about 0.4ml per gallon of water).

Equipment for Freshwater Dip

Equipment for Freshwater Dip

8.  Once the pH matches the tank (and the temperature is slightly higher), add the fish.

Angel in Freshwater Dip

Angel in Freshwater Dip

9.  Most fish initially stress, but then calm down.  Tangs are about the worst.  They play dead.  No kidding.

Purple Tang Playing Dead

Purple Tang Playing Dead

10.  Leave the fish in the dip for 5 minutes, unless the fish stops breathing or shows other signs of extreme stress.

11.  Move the fish over to the tank.

12.  Repeat as necessary (up to once a day for three days).


  • Seriously, tangs will play dead.  They’ll lay on their side, float lifelessly with the current, and try to terrify you.  Don’t freak out.  Watch their breathing.  If they’re breathing, they’re usually ok.  You may also find them stealing a peak at you.  If they’re watching you, they’re fine.
  • If you’d feel more comfortable, you can use RO or RO/DI water instead of tap water.  However, the pH is further from a marine aquarium, and this water will require more effort to reach a safe pH.
  • If you don’t feel comfortable with a freshwater dip, or if you think something is going wrong, just end the treatment.  Have a friend or your local club help if you wish.
  • Avoid methylene blue touching ANYTHING you don’t want to turn blue.  It stains just about everything.


Egekeze, John, Frederick Oehme, “Cyanides and their Toxicity:  A Literature Review,” The Veterinary Quarterly, Volume 2, Number 2, April 1980.

Kroupova, H., et al, “Nitrite influence on fish: a review,” Veterinary Medicine, pg 461-471, 2005.

Noga, Edward, Fish Disease:  Diagnosis and Treatment, pg 143, John Wiley & Sons, 2010.

Noga, Edward, “Amyloodinium ocellatum,” Fish Parasites:  Pathobiology and Protection, 2012.

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