To be able to check your fish, you should have a microscope or access to one. These photos may help you to identify some common parasites
There are many more such as Ichthyobodo a cool water parasite. We could fill pages on this site with them.
Remember they are caused by stress, including the dreaded water temp 52-56F, water sediments and water quality.
White spot full blown
If you see this amount under your scope take action at once . This is a purely a stress related injection. White Spot has no qualms about killing every fish you have. White Spot can be killed also by warm water but not many of our ponds reach 88F in our summers.
See our white spot page in the Health section of this site.
Fish Leech
See our fish leech page in the Health section of this site.
Dactylogyrus
Dactylogyrus need a temperature of 65F plus is an egg layer so treatment is based over 28 Days. Then Re-Check symptoms high gill rate, scratching.
Oodinium Pillilarus - Velvet Disease
Oodinium not a true parasite is more plant cells known as Velvet Fish has a (Brown - Yellow) dust over the dorsal area.
Ichthyophthirius - a swarm spore, also known as asymptornatic spore Trichodina spp. (stained) Childonella spp. (stained) Columnaris
Ichthyophthirius (White Spot) a most under rated killer in Koi do not look for white spot on the Fishes Body its mainly in the Gills. Jumping Koi don't jump through pleasure, they jump to rid themselves of the itching, scratching, scales missing & clamed fins.
Trichodina symptom a greyish slime appears over the fishes surface, flicking fins, scratching, clamped fins, laying on the bottom & off there food.
Childonella symptoms are as above, but can be a problem in cooler water.
See our Columnarist page in the Health section of this
site.
Body / Skin fluke
Body Fluke is a live bearer giving birth to live young does a lot of damage in ponds mainly in the spring time, if left unchecked will kill fish through secondary injections through the damage they have done through scratching. 
Fish louse
Fish louse symptom looks like a small transparent bead on the fish mainly on fin near the head the pectorals. must be dealt with without delay. This is becoming a real problem now - new cases this year up by 60%.
Read More Articles On Fish Health CLICK HERE
