I've got 8 rays in the same system and 4 in the same tank. One of my motoros has had disc rot for about 6 months I'll treat with salt and temp bump for a week to a week and a half it'll clear up and about a week later it returns. All other rays are fine with no problems. ph 8-8.5, water is hard, 0 ammonia,nitrite 0.1 mg/l, nitrate 10-20 mg/l. Rays are on playsand. Drip system at about 60 gpd through sediment and carbon blocks. Can't get any pictures because ray is covered in sand and you can't tell. Eats like a pig and is active. Is there any other treatments or anything else that would resemble discrot. Only around one edge on one side?
