Rainbow fish black chin

[FrostiesFishes]

Hello ive looked all over the internet and in my fish health book for help with this issue and ive medicated the tank with a lot of different meds. the water is good I've tested it multiple times over the last 2 months. I've got angelfish, corydoras , plecos and a leaf fish in the tank. i started with a school of about 13 dwarf neons and had no issues. then added some black marble angelfish that did go into a qt tank for about 6 six weeks and showed no issues. so i added them to the 72 bowfront. and slowly I'm losing 1 rainbow fish at a time. I've medicated with the aquarium co op trio as well as many other meds with no results. but its odd that whatever it is just killing one rainbow at a time. so it makes me wonder if its not the leaf fish attacking them. he is about 5 inchs and the rainbows are adults at about 2-3 inches. tho I've never seen him attack them but I'm wondering if it happens at night. no other fish in the tank are effected besides the rainbows and like i said it happens one at a time. i did have a person on a rainbow fish forum say they've seen what im calling the (BLACK CHIN) before but found no cause or cure for it . any advise would be apricated 

[Cory]

Water parameters are unknown, so can't go there. I would put it in Qt with 1 salt per 1 gallon and see what happens. could be something like TB which is incurable , but if the QT tank has good water parameters and salt like that, if it's bacterial or fungal , it'll usually get it.