As ubiquitous as dusts in air, fungi share the aquatic medium in which nishikigoi reside. Fungi and bacteria can become pathogenic once they gain entry via an opening such as a sore into a koi. Hobbyists have routinely focused their approach on anti-bacterial treatment when their collection of koi exhibits signs of infections. Just like bacteria, fungi can be equally deadly to an immuno-compromised koi. Although total recovery from topical lesions may take 1-2 months, the lesion by itself should heal within 10 days if appropriate measures were applied.
Open ulcers should heal within 10 days; however, regeneration of missing parts such as fins, tissues, and scales, may take months to complete. There are two agents that appear to be efficacious against fungal infections: amphotericin B and fluconazole. The drug of choice is fluconazole since it can be administered via the oral as well as the peritoneal route. Moreover, fluconazole is cost-effective due to its prolonged shelf life. Amphotericin B is difficult for the hobbyist to use since it's toxic to the fish's kidney (nephrotoxic). Mixed infections involving both bacteria and fungi can occur in a fishpond with stagnant water flow along with low oxygen tension. Therefore, dual therapy consisting of both anti-fungal and anti-bacterial agent becomes an empirical combination for severely sick koi. Fluconazole can be dosed at 6-mg/3 mL/kg given intra-peritoneally every 24 to 48 hours x 3 doses followed by assessment of therapy. Treatment may continue until lesions completely healed which may take up to 2 weeks. Once the koi recovers from its acute phase, the regimen of giving medication should be reduced to a 48-72 hours of interval. Therefore, do not inject the sick koi every other day for 2 weeks. The risks associated with frequent injections may outweigh the benefits. Fluconazole should include an anti-biotic agent at the outset for any major infections encountered in koi. Its compatibility with aminoglycosides (amikacin, tobramycin, gentamicin) allows solutions be mixed in a same syringe. Amphotericin B, on the other hand, is not so easy to use as it has limited compatibility, and relatively short stability once reconstituted with water. Amphotericin B is very nephrotoxic (hard on kidneys), so it is not feasible to add a second nephrotoxic agent to a treatment when an aminoglycoside (amikacin, tobramycin, and gentamicin) is already adversely affecting the koi's kidney function. Fluconazole should be considered when treatment failure occurs with a mono-therapy with antibiotics or when koi displaying morbid features as frequently associated with "dropsy".