First, just because your fish has gone off feed, doesnââ"šÂ¬Ã¢"žÂ¢t' necessarily mean it has a bacterial infection. Disinterest in food is one sign of bacterial infections - but it is also the first sign of most disease and/or stress, which can be brought on by all kinds of other things.
Do you see any other symptoms that would lead you to believe that you have a bacterial infection? And, are you water parameters in line and the other fish in your main display behaving normally? May sound like a silly question, but you may find treating with OTC to no avail if you haven't diagnosed the problem correctly. However, even if you have diagnosed the problem, adding it to the water is probably the least effective way to use OTC (oxytetracycline).
Not sure about the 10g active OTC per 181 g packet - I always thought Terramycin was 44% active OTC. Regardless, if the packet explicitly says it contains 10g of active OTC than you should go with this. You could try it at 10-12.5 mg/L daily, or at 50-100 mg/L for a 2-3 hour bath. Keep in mind that it will crash your filter if you use it for a prolonged duration. You are much better off to try and incorporate it into feed and giving it orally. The standard dose rate is 7.5 mg/100 g of fish body weight per day for 10 days. While you would have to guesstimate your fishes' weight and how much it would eat in a day and prepare food accordingly, it would be much more efficient and effective. Alternatively, you could try something like Seachem's "Focus", which is a liquid compound with a "binding" agent that also has contains an antibiotic (nitrofurantoin) and can be used to bind OTC, which is then added to some food. However, doing this is tricky, as you have to be able to measure very small amounts. Unfortunately, teaspoons and tablespoons just aren't going to cut it for measurements of mass (volume yes - but mass no).
Not sure if this helps, but should be food for thought.