I am quite sure that the problem is your PH. You probably know that the PH should be between 7 and 8.5. The difference between your 5 and the 7 required is 20 fold.
You Koi suffers from Acidosis and your koi probably has increased gill movement and an erratic swim pattern.
Bi carb will solve your problem temporarily, but the PH will probably decline again. You will have to also look for answers why the PH is low, and rectify the cause.
The low PH might be due to excessive algae bloom or an excessive plant growth that will take out the Carbon dioxide and Nitrates which will contribute to acidic conditions. A dirty pond will also cause a low PH.
Your long term solution: Add oyster shells and a few concrete building blocks to your filter, clean your pond and filter more regularly if there is any built up of sludge and increase your aeration.
Rectify your PH immediately with BiCarb, but remember to do that gradually. A sudden change will be more stressful for the koi than doing nothing.