He had an extremely serious bacterial infection, likely caused by aeromonas. Ulcerative disease in koi is most commonly caused by parasitic infections (flukes and/or anchor worms) combined with poor water quality and lack of suitable conditions. You did not mention specific parameters (this will help!).
Start by improving husbandry practices. Increase the water changes to 50-75% 2-3x a week, make sure the ph/kh/gh are sufficient, increase aeration/circulation and address flukes in both tanks by treating with praziquantel at 5mg/l for atleast 4 weeks. If another fish starts to show symptoms, start by treating everyone with a medicated food (such as MediKoi) and increase the salinity to .1%. Veterinary intervention is strongly suggested as injections of Baytril or another prescription a antibiotic has the best prognosis. Hope this helps!