Minnows dying

[Tarobot]

i've had six minnow for almost a year and have used them to cycle newly setup tanks but they have been in this tank for over four months. they have been slowly dying off one by one and i notice that the sickly ones have "black tire marks" on their sides. it's kept in a tank with blackneontetras cories and otos. the ph is about 7, gh and kh are relatively 80~100ppm. was wondering if anyone knew with the issue is, the side of the minnow has little black dots stretching diagonally across and it swims sickly heads up.

[Morainy]

Oh, I'm sorry to hear this. No, I haven't seen this problem before but the black dots make me think that it might be some kind of a parasite. Have you tried treating your tank with anything?

[Tarobot]

my heater was broken a week ago and i thought that one or two died from over heating but the tetras shrimp and cories in there have all been doing well so i wasnt sure if i should treat if im not sure whats wrong plus most medications just kill shrimps, i have added a bit of aquarium salt to improve gill function though.