If it's Velvet try Aquari-Sol with daily 50% water changes and salt. Velvet doesn't really cause lathargy though unless it has gone on for quite some time. I recently lost one of mine to velvet cause I didn't catch it soon enough for treatment, and it led to a major break down of his immune system. Try feeding frozen or live foods too. They should help build up his immune system. If you have substrate in the tank, and have a gravel vac, clean it really well with each water change till you eventually see no more junk getting sucked up. Really clean water will help too. Oh, I read on here somewhere that if you raise the water temp to around 86F it will get rid of it sooner cause the heat will make the velvet let go quicker and thus die. Whatever the problem is good luck!