The basics in your pond / fish disease tool kit include:

1. An anti parasite remedy for use in the pond water that contains Praziquantel (For flukes)

2. Salt for ciliated protozoan parasites like Ich, Chilodonella, Costia and Trichodina.

3. Formalin-containing compounds if the salt doesn't work (remove most of the salt first)

4. A chitin synthesis inhibitor like Anchor Control (ECORX) for Fish Lice and Anchor Worm

5. Medicated food for bacterial infections.

6. Sometimes the ability to heat a very weak fish in a small quarantine facility will save these weak fish. So having a collapsible tank that you can loan or rent them then sterilize later is nice, with a titanium caged drop in heater.