There are so many ways to prevent your fish from getting disease and bacteria infections. I would suggest doing the following ;

Don't over stock your fish pond, don't over feed them in the pond, always clean out debris from your fish pond, change the pond water very often and makes sure they are healthy in the pond.

There're are some antibacterial or antibiotics and anti fungi that need to mix to your aquarium to avoid that. I had the same problem before with my flower horn fish, one of my flower horn fish die because of bacteria.

oh. Really. Do you remember what is the name of the antibiotics>

Adding salt to your fish pond or fish tank would help to prevent both diseases and parasites in one's fishes. Please remember that it's not ordinary salt that needs to be added to the fish pond or fish tank, so be careful with the salt you are adding to the pond. I would recommend using aquarium non iodized salt.

@Martinsx You have a very good point with that statement on how effective the use of salt is in treating as well as preventing fish diseases and parasites. When you realize that there are parasites in your fish, a proper salt bath would handle the problem. I have lots of friends that own fish ponds, they always make use of aquarium salt to manage their fish pond and use it to treat and prevent disease in their fish pond or fish tank.

There are many treatments you can add in the fish tank, I add methyl blue in the water every time I change it, it helps to clean the water for any disease.

I think the basic way of prevention is having a well maintained filtering system. This helps a lot when it comes to preventing diseases because we all know that space is an issue specially with small tanks. I have a friend that have an isolation tank, where he keep some fishes with symptoms of disease. This way he can prevent it from spreading to all the fishes in the primary tank.

You should be very careful about water inside your fish tank or aquarium. You should always take care about pollution in the water. You should also install a filtering system in your tank/aquarium. Non-iodized salt can make water clean, remove pathogens, and prevent disease in the fishes.

I just use a bit of Methylene Blue with every change of the water. I reckon for the most part that is about all you need to use.