Parasites

Chilodonella is a ciliated ( having small hairlike organs) protozoan that causes infected fish to secrete excessive mucus. Infected fish may flash and show similar signs of irritation. Many fish die when infestations become moderate (five to nine organisms per low power field on the microscope) to heavy (greater than ten organisms per low power field).
Chilodonella is easily identified using a light microscope to examine scrapings of skin mucus or gill filaments.

This is the most common fish leech that is found in garden ponds. Whilst being a parasite it differs from Anchor Worm (Lernea) and Fish Louse (Argulus) as it only attaches to the host fish to feed, dropping off afterwards and hiding at the bottom of the pond in order to digest it's meal.
It can survive for long periods without feeding. Piscicola geometra is often introduced to ponds on plants.

Being clearly visible without magnification, it measures up to 25mm in length and appear to be pale brown, or brown and white striped, in colour.  Piscicola geometra is a strong swimmer and can move rapidly across the pond searching for it's next host.

Costiasis  or Costia  is a freshwater ciliated (having hairlike organs) protozoa (unicellular) parasitic species called Ichthyobodo necatrix. Formally known as species Costia necatrix.

These are stalked protozoa which attach to the skin or gills of fish to fed and can swim and attach themselves to other fish and then reproduces to infect other fish.
They are quite a common parasite in Koi or outdoor Goldfish.

Costia are sometimes found in small numbers on the skin and gills of fish without appearing to cause any harm. They live on cellular debris and the natural defence system of the fish semm to keep the parasite to an acceptable level.
However when large numbers establish themselves it becomes a serious threat and seemingly the natural parasitic behaviour takes over.

The Anchor Worm or Lernea is not actually a worm it is a crustacean parasite and is more commonly found in freshwater fish such as Koi. It has been given the name Anchor Worm as the head penetrates the skin and develops a shape similar to an anchor beneath the surface making them difficult to remove. Two egg sacks develop at the posterior end of the body. They eat away at the soft tissue and in doing so can cause secondary infections and fungus as well as the immense tissue damage. Vital organs may aslo be damaged depending on whereabouts on the fishes body the parasite attaches.

The parasite is quite large and easy to spot with the naked eye, however it is a tough parasite and, because of the way it attaches itself, can be quite a challenge to remove and/or eliminate.
Removal is desired however as leaving them untreated will mean that they successfully reproduce and will gradually kill off the fish around them. It is only the adult female that will attach to the fish. The male dies shortly after mating.

The fish louse (Argulus) is another common parasite affecting fish. It belongs to the class Branchiurans, which are ectoparasites of fish (any external parasitic organism). they live mainly in freshwater habitats and have flattened bodies comprised of five limb-bearing segments; the head has well-developed carapace lobes.
This flattened body means that they have a low profile when attached to the host fish but the adult Argulus can be seen without magnification, measuring up to 10mm in diameter.

Flukes belong to the family of a monogenetic named trematoda. These parasites have sets of hooks at the end portion of their body and with the help of these they affix themselves to
 the selected hosts. There are two varieties of parasites found on the bodies of koi fish, skin flukes (image on the right )
and gill flukes (image on the left ). The monogenean trematodas are similar to worms and can be 2mm long. An overstocked pool with dirty water is a perfect breeding place for this kind of microscopic intruder.
Skin flukes (Gyrodactylus) measure up to 0.4mm, and gill flukes (Dactylogyrus) are up to 2mm in length. Both types of Flukes can be found in small numbers on healthy fish, living on mucus and skin debris, without apparently causing any harm.