From the people I have talked too since it came out not many people rate melafix as an effective treatment ,
Looking at your koi , it looks as if your koi may have a fungal condition and tail rot however your photo's arent at all clear [could you re take them pleaseso we can get a better look]?.
Koi do not do well on their own so its always best to add a companion koi or goldfish for the course of the treatment, silly as this may sound it works , I know of alott of koi keepers who use the same trick when treating koi.
May I please ask how many gallons the container holding your koi is and exactly what filtration you have on it please ?
Would you happen to know your ponds water perameters Ammonia , Nitrite, Nitrate, Ph drop test kits not test strips which are highly suspect results wise , your best bet being the API pond test kit.
What are you planning to do with this koi, is it a life behind glass or in a pond ?
We kept our own koi indoors in a huge aquarium for 22 years before moving out doors so provided you have the right sized tank that ok ours was 6ft x 2.5ft x 2 ft so a life behind glass doesnt sound too strange to us .
Dave