Antimicrobials for Fish Food That Are Easy

Antibiotics - How Much to Glaze a Pound of Food In

Oxolinic Acid
At left, Oxolinic acid. It doesn't take much at all. 150 to 250mg per one pound of food. Measuring milligrams will probably require a very fine scale. 
Most of these scales for under 20$ will work fine. 
Reference 

API Fin and Body Cure
Active Ingredient: 250 mg Doxycycline Hyclate per packet. 
Dose: 4 packets of medicine glazed onto 1 pound of food.
Reference 

THIS IS YOUR BEST CHOICE OF THE CHART BY PRICE, SAFETY, AVAILABILITY AND EFFICACY. 

API FURAN-2,
containing active ingredients: Nitrofurazone + furazolidone + methylene blue
All of which are safe to be ingested by NONFOOD fish. 
There are 10 packets in a box. You need 10 packets per pound of food that you glaze. That's a lot of packets. Each packet contains 85 mg active furan. 
Packaging specifies bath treatment. 
Reference 

Confectioner's Glaze 
This is how we can make a medication remain on a food that's effective, easy, easy-to-get, and cheap. 

Here's how to use it. (Making Medicated Food)

Making food for Koi?
Use Blue Ridge's Immune Booster Probiotic + Food. See image at right. It's 3/16th inch, floats WELL and is well accepted. 

Making food for smaller fish? 2+ inches?
Use the Blood Parrot "Red" make sure it's the MINI pellet. This is all I fee my tropicals now. It's great for little Koi and any goldfish as well. Their mouths seem to have no problem with the mini. Even my 3/4" Tiger Barbs.

A mess of other "fish" antibiotics are available at ValleyVet supply they are very proud of their ability to sell antibiotics bypassing the requirement for prescriptions. They've got almost every common antibiotic available with a fish on the package so no prescription seems to be required. They are OFTEN out of stock. You can use dosing from this online reference.