First off may I have all your water perameters i;e Ammonia , Nitrite, Nitrate, Ph, Kh ?Next a little tip save yourself some money as pond salt is a rip off buy cooking salt instead.
Do not salt your pond, why because should you need to treat your pond some chemicals can react with the salt and cause you problems and if you have salted already and have to say use Formalin and Malachite with a Ph of 8.5 or above can cause the water to become Toxic to your fish.
Please note also Formalin should never ever be used with potasium Permangantate
If you must salt your fish it is far better to do a salt bath than add it to your pond why ? this is because salt doesnt evaporate and has to be removed by water changes.
Looking at your koi "which is a Shiro bekko by the way" we see that it is using all of its fins to maintain position and is not clamped, so it may well be a tad stressed by your adding of the gravel.
Personally I wouldn't use it in a pond because after a while detritus can build up and you'll need a pond vac to clean it....
This detritus can also hide some rather nasty things such as bacteria just waiting to attack any fish that is slightly off colour or has a breach to the protective mucus That protects it, which can cause no end of problems it would be better to take it out .
Looking at your pond it looks rather small and shallow how many gallons is it please?
Koi are the only fish that can out grow their suroundings so you may have to find new homes for them at a later date.
How deep is it please?, koi need a minimum of 4ft to overwinter safely .
How often do you do water changes on the pond ?
We do around a 30-40% waterchange and maintain our filtersystem on a regular basis you should be looking toabout the same.
If you have any questions no matter how silly you think they are please ask someone will always help you .
rgrds
Dave
