Fin Rot: Tetracycline question

I am pretty certain that a new fish I brought home has a severe case of fin rot.

Fins appear to be glued together and bloody. He swims in bursts and seems to have a hole in his cheek as well, on the left side.
(But he is a opaque white betta and many of them I have looked at on aquabid have appeared to have a hole or two on their face that other betta color types do not have.)

When he showed signs of clamping I used 1 tsp of aquarium salt + 1 half Indian Almond Leaf. (per gallon)
Symptoms moved fast even with daily water changes. I paniced and stopped the IAL and salt and used Betta Revive since he was in the stuff when I bought him. (It's the blue water in pet store Betta cups, it is fairly broad spectrum and says it controls a lot of different problems on the package.)

After using Betta Revive his symptoms didn't get any worse, nor did they get better so I just got a package of API T.C. Tetracycline which treats bacterial infections.

My question: The directions say to put one packet directly into the 10 gallon aquarium. I read somewhere, (either here on this forum or elsewhere on the internet) that someone had used a fish medication with similar instructions by making a concentrate by mixing 10 TBSP into a container with the packet and using 1 TBSP of the concentrate per gallon of water in the tank and otherwise following the directions on the package.

Does anyone know offhand the best way to keep the leftover tetracycline as fresh as possible? With the size of my q-t tank only one packet will be needed to complete the full coarse of the treatment as it calls for 25% water changes + a dose of med.

In the event no one here knows do you think a pharmacist would know? Or will they freak out on me for asking them about "fish-grade" tetracycline?

Should I just try to find a darkish container and keep the unused medicine in the fridge and start treatment today? I doubt I will be able to reach a pharmacist on Easter.

BTW- I can still fill out a sticky if anyone insists but the info in it won't add much to my question.