Is there any chemical/medicine for freshwater/tropical fish that will boost their immune system? neon tetras?

so i have an aquarium that i kept a sorority of bettas in (all females with plenty of places to hide, don't lecture me, they never got in fights) over the past few months i am down to only one female, and i'm concerned theres something theyre not getting that's lowering their immune system..i keep their stress down to a minimum by covering the output of the filter with plants and such to reduce water current and feeding them a variety of foods i can get my hands on (organic peas cooked and chopped up, frozen blood worms and brine shrimp when i can along with their main feed source of freeze-dried bloodworms. i add the water conditioner whenever new water is added to their tank. the majority of deaths my fishes come to is from fin rot, but i just lost a crowntail that seemed healthy, but in her final days was swimming weirdly and beggining to linger in one spot for hours and hours. the remaining fish appears healthy, her color is as vibrant as when i bought her and she eats well and swims normally, but i wanted to add some tetras or glofish to the tank, probably just a small school of neon tetras after i give the tank a thorough cleaning to get rid of any bacteria/disease that may be there from the dead fish (i always tried to quarantine sick-looking fish so they wouldent die in the tank with the others, but the crowntail died soon after showing symptoms of ill-health) anyways, is there any sort of chemical or something i could add to the water to boost the immune systems of the bettas/tetras? would adding a live plant help? if so, what kind?