If you're visiting this page with healthy bettas, congratulations, because you're thinking ahead. Betta diseases can spring up out of nowhere, and so reading this page and having a little betta "first-aid" kit will come in handy when the unexpected happens. If you're reading this page trying to find solutions for a sick betta, don't panic yet. Read for what seems like an appropriate medication, and then go to the pet store. You might also ask a knowledgeable sales associate if the medication you're selected will help. Also, scroll down past the medications for descriptions of betta diseases.
What Should Be in Your Betta First-Aid Kit
Hospital tanks: with any other fish that live in groups, you'd want to isolate a sick fish into a specialized hospital tank. This makes dosing medications a lot easier. It also decreases the chances of getting other fish sick. That said, bettas already live alone so their bowl can act as their hospital tank. But if you are for some reason keeping bettas with other fish, like having a large school of female bettas or keeping it with an African Dwarf Frog, etc., segregate the fish immediately. This hospital tank should be kept very clean, and this may require full water changes every two or three days. Keep in mind that the medications you dose will probably have instructions as to when to change water. Also know that with some sicknesses, water changes can cause stress for fish, so be ginger and don't overdo it. Remember to always use water conditioner.
Heater: You should already own a heater for your bowl (this one looks a tad different than it does in the site of the link, but it's great. Also look at this one here). You'll want to keep your hospital tank very warm, as this will accelerate the life cycle of parasites and kill them faster.
Aquarium Salt or Rock Salt: this is something everyone should own. I use tiny amounts every week when I clean the bowls because it acts as a de-stresser, but it's
also great for preventing disease and encouraging fin regrowth after fin rot or other damage. When a betta has a fungal infection, salt can be used in a salt bath that can kill the fungus and dehydrate it. To make a salt bath, mix one teaspoon of salt per gallon to the betta's water for 15 to 20 minutes. Then do a water change.
Epsom Salt: This is a great product to help ease betta constipation and can be found in the laxative section of your closest drugstore. Use a tablespoon of salt per gallon of betta water for 15-20 minutes, just like the aquarium salt, as a salt bath.
Aquarisol: a wonderful medication used in the prevention and curing of Ich, velvet and other parasites. It uses copper (see "copper-based medications," below) and so must be used in correct dosage.
Melafix: If your betta has fin rot, use this after the bacteria which causes it is gone. It has Melaleuca oils which, like aquarium salt, help with fin regrowth. Melafix does not treat the bacteria which cause fin rot, but is only used after it is gone. Keep in mind that if you're advised to use the medication Bettafix, it's just a less-concentrated version of Melafix. Especially if you're trying to make fins grow back after a serious injury or fin rot, I advise using Melafix over Bettafix.
Fungal meds: There are lots of options for treating fungal infections. One that I've used for minor fungal infections is Mardel Maroxy, which is a liquid dropper medication. It's worked well when I've seen faint signs of cotton-y patches on bettas.
Parasite meds: You have lots of options here too! I haven't dealt with parasite infections directly yet (thank goodness!) but if I were treating one, I'd certainly be feeding Jungle Anti-Parasite Medicated Fish Food to help the fish fight the infection from the inside too.
Jungle Fungus Clear (Blue Tablet) or Jungle Fungus Eliminator (Yellow Crystals): These antibiotics treat small bacteria infections, not fungus, ironically, like Fin Rot
and Columnaris. Add one tablet per 10 gallons of water or one teaspoon of the crystals per five gallons of water.
Broad Spectrum Antibiotics: Several medications work on the two types of bacteria that cause betta diseases like Fin Rot, Septicemia, Columnaris, and sometimes Dropsy. Try Kanacyn, Tetracycline, or Maracyn I and Maracyn II (use I and II together).
Methylene Blue: If you bought a betta in a bag or cup that had blue water, the blue came from this drug. It is an active fighter of fungal infections and so keeps betta cups cleaner and healthier in the stores. If can also treat Ich. It doses with one teaspoon per 10 gallons or 12-14 drops for one gallon. It is also good for sterilizing betta equipment: if you have multiple fish that share equipment, make sure you are sterilizing it with this medication. This medication can't be used in a tank with a filter as it will be filtered out. It is best used in a small bowl (1/2 gallon or so) for short treatment sessions.
Copper-Based Medications: This has to be used carefully, as copper is poisonous to bettas and hence must be dosed properly.
Copper can treat parasites like Ich and Velvet, and some meds come in a safer-copper form.
BettaMax: Great all around preventative. Very effective against velvet & clamped fins. Use anytime you add new fish or acclimate.
Thank you to this betta first aid site, which provided much of the information I've found.