The first step in using Formalin is to determine if you absolutely must use the Formalin. The principal reasons I would use Formalin would be if I were confronted with a mixed infestation of parasites, or if I was dealing with Costia, whether mixed with other parasites or not.
Once you diagnose Costia, you have only a few treatment choices. Formalin is a good thought, but a formidable medicine. And if you've discovered Costia with other parasites, you can suspend your biopsy effort and get busy; you should focus on Costia. One might ask "What about the other parasites? Why aren't you doing more biopsies on more fish?" The answer is that once you have found Costia on some fish, you know you have the worst parasite there is, and you're called upon to use the most aggressive treatments, and, "Anything that nails Costia will nail all the rest."
So you have decided to use Formalin
First, accurately calculate the volume of the system. Be sure to do this accurately, and double check your calculations. Never take the word of the water meter or the pond owner from memory. Use both dimensional calculations and a water meter for greater confidence.
Using the system length in inches, times system width in inches, times system depth in inches will give you a sizeable product, this product can be divided by 231 to give you US gallons in a system.
For example; a system sixteen feet long, x 30 inches wide x 20 inches deep, the volume is 469 gallons.
Second, calculate the amount of Formalin you need:
There are differences in the amount of Formalin you'd use based on the species and age of the fish.
For Goldfish or baby fish, 25 ppm is recommended.
For young (not baby) to adult Koi, 50 ppm is recommended.
For the purposes of this article, we will discuss the use of Formalin 37%.
For 25 ppm you would apply 37% Formalin at a rate of one milliliter per ten gallons of water to be treated.
For 50 ppm you would apply 37% Formalin at a rate of two milliliters perten gallons of water to be treated.
Because the Formalin will kill a large percentage of your biofilter's beneficial nitrogen removing bacteria, it's crucial to remove bio media to a place where it can be kept cool, and wet, or leave the media in the filter and bypass your filtration system so that the beneficial nitrogen reducing bacteria aren't killed.
It's crucial that you are aware that if you bypass your filter, you STILL NEED AS MUCH CIRCULATION as you possibly can get. If you do not maintain plentiful water circulation and aeration, your fish *will* die.
This is because Formalin consumes a considerable amount of dissolved oxygen at it's application. In fact, in a test tube without agitation, every five ppm of Formalin consumes 1 ppm dissolved oxygen. You can offset or ameliorate this effect with aggressive circulation or aeration of the pond during treatment.
Also, when you pull your bio media out of the filters, you will stir up a cloud of debris. This must be cleared or allowed to settle out, or this brown cloud must be siphoned off before you apply Formalin as Formalin is easily bound up by organic solid debris. So the clouding should not be present when treating with Formalin.
Finally, if it's possible, remove all your plants from the system to be treated. This is because the plants, and their soil will bind and inactivate the Formalin to an unfortunate degree, perhaps sparing the parasites.
You might develop a reasonable concern that if you bypass your filter and remove your plants and all that; parasites will remain alive in the soil, plants and filter. This is true to a limited extent. But keep in mind, since you will replace your plants and resume filtration before all last traces of Formalin are gone, these areas will receive SOME formalin exposure.
PLUS, and perhaps most importantly, it's an essential part of the regimen we're outlining that you will treat several times, meaning; at least three times over five days. On these subsequent treatments, you will eliminate parasites which might have been "hiding" on plants or soil the day before.
Here's a pretty good regimen: Treat with Formalin on day one, day two, skip day three, and then treat again on day four. If you want to be VERY confident of total parasite annihilation, you would also repeat Formalin treatment on day six.
This regimen is pretty good for Costia and very good for Flukes.
How exactly to treat:
Mix the calculated dose of Formalin in a bucket of pond water and distribute evenly through the pond after bypassing the filter and removing as many plants as possible. Maintain high aeration and circulation.
Wait exactly 120 minutes and then dump 70% of the pond water. Then dechlorinate and refill the pond. Replace the plants in the pond and restart the filter.
This water change, at that exact time, is crucial because UGA researcher Dr John Gratzek showed that after two hours in Formalin 50ppm, all the parasites you intended to kill off are gone (for all intents). So the fish should NOT be made to suffer past that time in the Formalin unnecessarily.
Finally, what temperatures would you (or would you not) treat in?
Cool water is fine if not ideal, as long as it's not 'near freezing; because in very cold water Formalin tends to 'sink' and is reported to form Formalin hotspots...I've never had such trouble.
Worse, though, would be to have to treat fish in warm water, that means water over eighty degrees Fahrenheit. If you need to use Formalin, and your water is pretty warm, I'd recommend that you change half the water out and allow the new tap water to cool the pond a little before applying the Formalin.
Yes, as you can tell, if you're cooling the pond with a water change, then doing Formalin treatments which merit repeated water changes, you're going to get love letters from your water authority.
Advanced Aquascape™ pond, two-belly design, 6x6 upflow aggregate filter w/ 9,000gph pump feeding falls.
Also, some people talk about 25 ppm "Leave In" treatments. This is how it was done up til a decade ago. People would apply 25ppm and just leave it in the pond or tank. The Formalin would dissipate on it's own over time. The parasites were annihilated but most often, the fish were also burned.
I don't like "Leave in" treatments.
However, there are *two* times when you need to modify the dosing regimens above.
First, if the pond is quite large and a water change is time consuming, you may consider treating the pond with Formalin AFTER you drop the water level 50%. Then depending upon the age and species you're treating, you apply 25-50ppm to the remaining half-volume water. Wait 60 minutes and ferociously refill the pond, cutting the strength.
Secondly, I have recommended the 25ppm Leave In regimen in very large ponds. This is a poor second coice to the water changing method but sometimes a pond is so large tha any water change would defy practicailty.
Good luck -