He could benefit from antibiotics, BUT, dosing a display tank with antibiotics is unwise. Were his eyes cloudy when you got him? Aside from squid what else is he eating? Define shark food? How often are you feeding?
Try getting either Zoecon or Selcon and dosing the food one time a day with it, then feeding him. Also start using FRESH minced garlic with the shark food. Fresh means right from the
raw garlic glove,
not anything in a bottle nor processed from the grocery store. Fresh garlic has a natural antibiotic, allicin. I would offer several smaller meals daily that is dosed with the garlic and garlic juices. Try to get the smashed up garlic, retaining the juices, with some meaty food then IMMEDIATELY syringe feed your shark with this. The quicker you can feed the shark with the smashed garlic and juice with the food, the better. Allicn in garlic quickly becomes ineffective once the garlic is opened up. Don't offer the zoecon/selcon with the garlic. Offer one small meal with the supplement, then other small meals throughout the day with the garlic, using a syringe for target feeding the shark.
What is your water source? What are all your water parameters?
https://forums.saltwaterfish.com/c/index.php/articles/content/104
Additionally, start doing 5 gal water changes daily for the next week. Be sure that your water is well mixed and stable over 48 hr period using a pump and testing for salinity and pH prior to the change.