Help possible Velvet

Hello,

Some of my 6 week old Notho fry are showing strange colouration. Some females have light and dark bars when compared to the normal females with 1 overall body colour. Some males and females have a spray of black 'dust' on their back and head area, sometimes extending to the gills. Their behaviour seems normal. I always thought the teaspoon of salt per gallon was a bit much and used much less, but now Ive doubled my salt dosage with water changes, not knowing how much salt is in the water ATM. The tank isnt spotless and Im hoping I can use these fish to breed in the future.

Thank you,
Mitchell

 

I seperated the males and females into clean 20L tanks with tap water and some Sera Aquatan. On one site I read that water movement, clean, warm water and low light conditions can help to combat velvet, but as the prof said Nothos dont live long in warm water. Each has a airstone and heater set at 25 degrees celcius, also, 1 teaspoon of salt per gallon. The water has a neutral ph. They all seem to have clamped fins now and struggle to swim in the gentle current of the airstone. This started after I moved them to these tanks, is there something wrong with there water parameters?
Please help, thanks a million