IMO if you want to stimulate the immune response of your fish, and keep it healthy long term.
1. Feed a high quality, highly digestible premium diet to your fish where all of the nutrient levels exceed the minimum requirements, including all of the various vitamins and minerals. While ingredients such as Spirulina, Garlic, Seaweed, Micro-Algae, and even exotic ingredients such as Ginseng, are not considered true probiotics, as they are not "live" organisms, they do in fact contain bioactive compounds, that have been proven to have a probiotic effect on fish. Some of these compounds have been shown to have biological effects in fish such as growth promotion, immunostimulation, anti-stress, anti-bacterial, anti-fungal, anti-virals, and appetite stimulators.
2. Keep your water and filters in proper condition (limiting the number of pathogens) If your stocking density is high,
then be prepared to perform the required maintenance to keep conditions stable, and water quality high.
3. Keep the overall environment for your fish as stress free as possible.
 
While I applaud the commercial aquaculture circles for seeking alternative routes to treating fish pathogens via more natural methods, the reality is they are now only doing so due to their own large scale and long term abuse of medications such as antibiotics. Anyone that has read my write up on treating internal protozoans (spironucleus sp) via an oral 3% magnesium sulphate solution knows that I am all about using alternative treatment methods whenever humanly possible, but in the case of probiotics I don't believe that hobbyists need to supplement live microorganisms in order to keep healthy disease free livestock. Simple husbandry practices such as listed above have served most of us very well over the years, and I expect will continue to do so for many years to come.
If there's anything else that I can assist you with, please don't hesitate to ask.
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