Prophylactic treatment with chloroquine phosphate. This is for ectoparasite infection (cryptocaryon especially), but not tolerated well by all species. So read this v first!
Chloroquine Phosphate (updated 9-06-2023) What It Treats: Marine Ich (Cryptocaryon irritans), Marine Velvet Disease (Amyloodinium ocellatum), Brooklynella hostilis and Uronema marinum. Please Note: Chloroquine is unlikely to eliminate internal (or intracellular) Uronema. How To Buy...
humble.fish
Some would recommend a separate tank/new water, prophylactic treatment with antibiotic(s), possibly with a fungal treatment regimen.
FWIW, I have used these in combination -
Maracyn + Maracyn 2 + Maracyn Oxy provides the triple effect against gram positive + gram negative bacterial infection, and + fungal infection, all at once. These can prevent a super infection following the above ectoparasite treatment regimen.
Alternatives involve isolation, with or without UV. I’m a fan of UV, personally. Just not during medication as it tends to destroy the chemicals being used.
This is a short overview and there is a lot more to it. Starting out, you can also buy specimens pretreated online which may be a better way to go, until you learn more. It costs more up front, but usually saves a lot of heart ache and expense further on!
Also, learn about and adopt hygienic protocols for isolating your system from others systems. When buying specimens or rock or even used equipment become aware that they can all potentially become vectors for pathogenic infection from another infected source, regardless of how careful you are otherwise!
Just my $0.02