Demystifying Freshwater Dips

Lately, there has been a lot of talk about treating new fish with cocktails that contain multiple medications and chemicals. So, I thought it would be a good idea to cover the most straightforward treatment that everyone has on hand: freshwater. That’s right, you can safely treat your marine fish with freshwater, and I’m a huge fan of it. It’s gentler on the fish and surprisingly easy to do. 

Before I get into it, let me make clear that this is not meant to be a cure-all. Proper quarantine of all incoming fish is still highly recommended, and additional treatments with hyposalinity and/or medications may be required. A freshwater dip should be considered the first step in the quarantine protocol. Now that we’ve covered that, let’s start from the top.

You’ve just brought home new fish from your local fish store, or the overnight package of fish has arrived from your favorite online vendor. The first thing you want to do is to visually inspect the fish through the bag, making note of fish behavior (rapid breathing, flashing, etc.) and physical appearance (lesions, pop eye, obvious parasites, etc.). Assuming everything looks okay, the next step is to float the still-closed bags in your display tank or sump to temperature-adjust the fish to your system. While waiting for this to happen, you can prepare your freshwater dip.

Preparing the freshwater dip.

I use regular tap water that I dechlorinate and temperature & pH match to my system. Tap water is safe and bacteria-free, and it’s easy to adjust the pH. I do not recommend using RODI water for this dip as it’s difficult to adjust the pH, and depending on your water storage capabilities, it takes time to make. Tap water is effortless and can be done in seconds. Adding an air stone to the freshwater dip will not suppress the DO. And that’s it; it couldn’t get any simpler. 

Acclimation:

Many people perform the drip method to acclimate their fish to the system water (if you don’t know what this is, great!). I absolutely hate this concept and prefer to immediately transfer my fish to my system water in a quarantine tank or bucket. As soon as you open the bag, the CO2 off-gasses. This converts the ammonium (NH4+) in the water to ammonia (NH3), the former being harmless to the fish while the latter is toxic and will quickly burn their gills. The longer the fish stay in this high ammonia water, the more they’re being hurt. Think of it this way: if you’re in a pool with a high acid concentration, would you rather someone drip water into the pool for a long and slow reduction in the acid OR remove you from the acidic water and place you in clean and safe water? I won’t judge if you’re in pain, but I’d much rather take option #2 and think the fish would, too. 

The dip:

Now that the fish are temperature acclimated, you’ll need to assess whether they’re in good enough condition for a dip. If they seem highly stressed, giving them a few days in a quarantine tank is perfectly okay, letting them rest and adjust. Once ready for the dip, you can safely add them to the freshwater dip without a slow salinity acclimation. Most saltwater fish can tolerate a drop in salinity extraordinarily well. I would only add as many fish as you can keep a close eye on; you’ll want to watch your fish’s behavior and look for any parasites that fall off the fish. A dark-colored bucket or tank and a flashlight will allow you to see the parasites more easily. I generally keep the fish in the freshwater dip for around 4-5 minutes. While some larger fish can tolerate a much longer bath, I haven’t found many benefits in going past the 5-minute mark. I have yet to come across a fish that can’t tolerate a freshwater dip, but that doesn’t mean there aren’t any, so I’d exercise caution for naturally skittish fish. After the dip, you’ll want to put the fish into water from a quarantine system. If you rested your fish in the quarantine tank prior to the dip, you’ll want to clean the tank before returning the fish to it to avoid reinfecting them. 

Flukes on fish https://www.cabidigitallibrary.org/doi/10.1079/9781800629127.0017

Treatment:

What you observe during the freshwater dip will determine whether or not you’ll need further treatment. Treatment options can be as little as hyposalinity or require medications. I tend to favor milder treatments and reserve the harsh chemicals and medications until absolutely necessary, so I’ll start my quarantine with hyposalinity for 2-4 weeks. You’ll encounter many afflictions, but the first one on my list is also the most common parasite you’ll come across: Neobenedenia (flukes). But that’s for an entirely different discussion we’ll dive into another time.