Flukes, whether they are on the gills or scales, are a group of parasites that can all be treated and identified by the same means. Unlike other diseases that affect freshwater fish, flukes can infect not only fish with a weakened immune system but will infect the healthiest fish as well. These parasites are roughly 1 mm in length and are long flatworms that infect the fish throughout its entire body (hence why it goes by so many different names since each location it infects represents a different type of parasite from the same family).
Since flukes are extremely small in size, it can be hard to see them on the fish's skin or near their gills. There are some giveaway signs that something is wrong with the fish, but these are the most common traits of other more popular diseases that fish can get. Some of the common symptoms that an infected fish will show with flukes are the fish scratching itself against objects inside of the water, mucus on the gills or its body, sunken stomachs, rapid breathing from both the mouth and gills, and lastly a redder colored appearance on their body. The red color against their body is mostly due to the fact that the fish is trying to itch the location of where the flukes are on the substrate to remove them, and is commonly seen as the fish twitching around when it becomes extremely itchy.
If you can get a magnifying glass, you will be able to see black dots on where their eyes are located (along with movement if you can watch long enough and the fish isn't darting around).
The life cycle of these flukes does not have a secondary or previous host, thus making it extremely hard to eliminate if the whole water is not treated. Since they can breed and spread with ease from one fish to another, having a closed system such as an aquarium or pond can allow for these parasites to live there indefinitely by always infecting new fish. Sadly, these parasites can come from frozen or live foods that are not gathered from a reputable food maker.
The rate at which they reproduce all depends on various factors such as the temperature of the water (the warmer the water is the more they will reproduce as their life cycle gets sped up), the water quality parameters (parameters that are out of the acceptable range will allow for the parasites to reproduce and grow faster as they can infect fish much easier), and also what type of fish are being held inside of the water. Many commonly associate this type of parasite with poor water as it makes more of an appearance in poor water conditions since the fish's immune system will be weaker and it will show other symptoms as well.
Treatment and Medications
Since these parasites are extremely small and can infect any type of fish regardless of if it is healthy or not, the treatment must be given to the whole setup with all of the fish inside. Some of the more common treatments for this disease can include organophosphates, praziquantel, mebendazole, and toltrazuril. Other treatments that are known to work include salt baths for each fish individually, or by using potassium permanganate in a solution of 10mg per 0.26 gallons of water (10mg per 1 liter) for about 30 minutes.