Formalin in Koi Ponds, Fish Tanks How to use Formalin:

Try not to leave Formalin in a system unless you have to. Don’t use formalin in very warm, or very cold water. Don’t use formalin unless you accept the consequences, or unless you have expert assistance on hand.

How to use Formalin:

Formalin 37% should be used as a long term treatment for ornamental fish to clear parasites. It has been suggested in the literature that this can be left in a treated system indefinitely. The dose is 25ppm and that is achieved by using 1 cc Formalin per 10 gallons of water. This can be left in the system indefinitely, but that is not how I would treat my own fish.
Research has shown that the fish need only be in the Formalin at 25 ppm for a mere 2 (TWO) hours to benefit from the parasiticide. I usually recommend that folks leave the formalin in for 6 hours, then do a 50% water change and replace the filtration to the system. Give the fish a day off, then repeat the treatment, two more times, total treatment period is 9 days. (Here is a proprietary Formalin, please use it per label instructions)

Simplified:

Day one: 25ppm Formalin added to the system, after filters have been removed to isolation.
Day two: (12 hours after primary treatment) Large water change, then the replacement of the filters into the system.
Day three: No treatment
Day four: Repeat day one. And so on for three rounds.

Formalin has also been used as a dip, prior to, or after the sale of a fish. I discourage this. Doses of Formalin required to clear surface parasites in short periods of time present a great stress to an already stressed specimen. Gills that may have been damaged by pH and ammonia stress in the shipping bag cannot easily adapt to being further burned by formalin. In truth, the treatment is fairly effective, but salt dipping is sometimes preferred, and less toxic/caustic to the fish.

If you want to use Formalin as a dip on a fish that has NOT been shipped or severely stressed, I would recommend a mere 50-100 ppm (2-4 cc per 10 gallons) for no more than 1 hour, much less if respiratory difficulty is seen. This is stressful and may singe the fin tips.

Common Questions About Formalin:

Question: How will Formalin affect my filter? I cannot remove the media!

Answer: Formalin may kill alot of the filter bacteria, causing ammonias, and then nitrites, to accumulate in the system. It depends on how long the system has been operating and how much ‘filtration overkill’ there is. One thing you can do to reduce this is to remove the ‘stacks’ from the under gravel filter, or in some way abbreviate flow of treated water through the media, and also to abbreviate feeding to one, small feeding per day. One problem with this: If you are combatting a pathogen like trematodes (flukes,) then the obstruction of the flow through the gravel may spare the filter bacteria, but also may spare the lives of the developing eggs or embryos of these parasites. This is the reason for repeating the treatment cycle three times. To intercept the different life cycles of parasites that may have survived the previous rounds.

Question: What signs should I look for to tell me the fish are in trouble?

Answer: Signs of respiratory distress are suggestive that the fish are in trouble. You should do an immediate, partial water change with dechlor, or better still, remove the fish to a remote system at the first sign of real gasping. The fish do not have much time when hanging on the surface, and there may have been some miscalculation in the system volume, the dose of formalin, or the temperature may not have been considered.

Question: How toxic is formalin to me? How long does it last in the system if not changed out?

Answer: Formalin is a known carcinogen for humans, if contacted long enough. It is also a skin sensitizer, causing eye and throat irritation when in contact with these surfaces. Obviously, you don’t want to drink it. Formalin’s dissipation from a system is governed primarily by the amount of organic material in the system, in other words: In a glass jar of clean, non-agitated water, the formalin would stay in solution for a long time, but in a mud bottomed pond, would be bound out within 2 days.

Question: I heard that Formalin cannot be used with salt. If I treated with salt first, do I have to remove the salt before using Formalin?

Answer: I have treated a large number of fish with salt. In two years of practice, but I have only used not quite one gallon of Formalin. Does that tell you something about the utility of Formalin, when salt is available? To answer your question: EVERY time I used Formalin, (and this would be echoed industry wide), I have used it concurrently with Salt. No problem. I am NOT saying that the combination is safe, I am just saying that I have ONLY seen this problem in print, not in practice. There is a difference.

A theory exists that in very hard water and water with high carbonate alkalinity the combination of salt and formalin was toxic in some author’s hands. I have not had that problem. Still, some judgment is needed and I might recommend a partial water change to remove some of the salt, but just as importantly, to cool the water before applying any formalin. Do not use Formalin in water over 78 DF.

Question: When DO you use Formalin Dr. Johnson?

Answer: In ponds and tropical fish tanks I kind of like it. It’s clean and quick and I always have more filter surface area than I need to killing off half my filter bacteria still leaves plenty.. I would use it when the client had a priority on plants, and could not, or would not use Salt. I would use it when I was combating Flukes and did not want to use an organophosphate or praziquantel. I would also use it if I wanted to clear Oodinium, which is often salt resistant, but only if the fish was not too badly eroded, dermally. I would not use it when the fish were ulcerated.

Enter your email address for a free PDF of this article including its images.

Enter your Email Address

My Favorite Amazon Recommendations

Items and content will not show in "Reader View" check your browser.

This is The LCD Screen Scope
We Did The Tutorial With. I did a twenty page tutorial (Here's the tutorial) with video, audio, images and even little parasite movies to show you how to use a microscope. 

This Filter is Good For BIG Dirty Tanks
I've been wrestling with water quality with the turtles. The sponges work GREAT, but in THOSE turtle systems where they eat MICE the poops are bulky. This filter can handle it. In "Tortu's" tank, it's got an 800 gph pump and it rocks. 

PraziPro for Flukes
They nailed it. Figured out the solubility and worked out the dosing. It works. 

Buy Some Good Koi?
No, this isn't really for the 'high end' collector y'all. But for someone who isn't near a decent garden center, here are "good-to-quite-good" Koi you will like at a very good price. I know these guys personally.

Best Food, Ever
It's made for (and I discovered it for) my Blood Parrots but the small size, intense color enhancers and excellent formulation make it superb young-Koi food. Oh, and it's AMAZING on color-cichlids like Flowerhorn and Blood Parrots. 

Pro Air 60 Aerator is a VERY high output air pump pushing my whole fish room (17+ drops), and I have one at home driving everything there, and I have one as a back up. 

Formalin Malachite (Not dilute)
There are formalin malachite preparations at 10%, 22% and 37%. There's economy in the concentrates. Hard to get Prime shipping because air transport is curtailed. This is a good value on 32 ounces. 

Topical For Koi and Pond Fish Ulcers
I like 7% tincture of iodine because it stains but it's hard to hurt anything with it. Used with a WOVEN gauze, this works well to clean and disinfect a wound. Only use ONCE. Do not 'repeatedly' scrub wounds. 

Confectioner's Glaze 
Is the way to bind a medication to fish food. Gone are the days of paste food and oil. The write up is done, it's RIGHT HERE.

Koi Health & Disease
Hopefully this link takes you to the newest edition by PRIME to get it to your house the fastest. 

I have over ten of these Titanium Heaters in my fish room and at home. They're a paradigm shift in aquarium heating. They're titanium and 400W for under $30! Whaaaaaaaaat?

CyroPro by Hikari is safe and easy for Anchor Worms and Fish Lice.

Whatever heaters you use, back yourself up with a temperature controller, it'll turn on, and off your heaters. If your heater seizes "on" at least the thermostat will stop a tragedy. 

LifeGuard by Tetra is a tablet version of a tame 'chloramine-t' or 'halamid' compound that's easy to get and good on bacterial infections, in baths. 

Antimicrobial
If you're making medicated feed for a larger group of fish, this will come in handy. Dosing is available in the site. 

For me, filtration-of-choice
For aquariums, even small Koi holding facilities or small ponds. These sponge filters are cheap, easy to clean, and they clear hazes very well when used with some Accuclear or similar.

Rubber sided, round, nettable tanks
Make great hospital or quarantine facilities. They SHOULD cost about $200-300 depending on size, but this, lower quality unit (while panned in some reviews) may be good. Don't overfill and make sure it's propped up.

1200 Watt heater, on a thermostat held securely above the waterline, works GREAT as a large tank heater. That's all I'll say because there's risk of electrocution and all that. But then, even in a bucket you could get killed. Sound like fun?
I have several of these ALLIED PRECISION stainless ones that are a couple years old. Working still. 

Potassium Permanganate 
500 grams could be a lifetime supply but it's 50% more than the 100g cost wise, for 500% more amount. Dosing is in the site and the book.

What Does Ajax Eat?
I looked for something well formulated, with meat as the first ingredient. Something UNDER $2/lb and something they could deliver for free. And this was it. He looks and feels great on it.