Bacteria and viruses are the other common threats that will also compromise the health of your fish. Basically there are few things which you can check. First are the gills of you koi. Watch carefully and determine whether there’s any sign of swelling. Sometimes upon close observation, you could even see growth of white cotton moth on the plate surface and these are mostly associated to fungus infection. A normal healthy color of the gill should be slightly reddish and even appearance of pale pink is not desirable. (Refer to attached photography shown above). Another common routine checkup, which you can perform, is by looking at the fins. Make sure that the fin structures are not frail still intact without presence of white dots. If there’s any, observe whether the koi will show signs of scratching or flashing on the pond surface, if it does, then most probably it could be sign of unhealthy development. See article on treatment of ich/ick or white spot disease to understand how to deal with it.
The health of your koi can also be monitored by just checking the underside of the fish. Usually what you can do is by observing for any stationary fish and then by carefully using a fish net and together with both hands, flip it around in order to observe the underside. This will require some gentle handling because koi can wriggle and flip around splashing up the water and if you are not careful, you might even slip and drop the fish. If this becomes a problem, you can either place it into a transparent plastic bag in order to observe it carefully. Basically what you are looking for are signs of bloated belly which could mean, an early warning towards development of pine-cone disease sometimes referred to as swim bladder disorder. Best treatment approach usually is by using prescribed antibiotic dose best recommended by a qualified veterinarian.
Check out on the list related to basic fish medication.