Help please. My Australian Rainbows seem to have developed an illness that doesn't have obvious visible symptoms. A female died last week after nothing much looking physically wrong. She started sucking at the top of the water and stopped eating. I've experienced ich, fin/tail rot, columnaris, and other hideous fish diseases in the past and what the Rainbow has isn't anything nearly so visible. Apart from looking stressed and emaciated the female that died seemed to have a small, round and not very pronounced lump on her side. Almost like a tiny tumor. Scales didn't appear to be missing in this spot. I'm writing because my younger male Rainbow has now stopped eating and started hiding in the plants more often. He has no "tumors" or other visible bumps. The only thing I can see different about him are two small scale-sized "highlights" that are consistent on his left side. They don't appear to be like "strings" (unless they are extremely short strings). Here is my remaining healthy rainbow fish for comparison (he was always bigger since he's a bit older). Could the sick fish's gill area be slightly more pink than the healthy one? I think it might just be a trick of the camera because any pinkness isn't obvious in person. Other deets: 35 gallon tank There haven't been any additions of fish to this tank in months (lets say 4 months) I do a 20-30% water change weekly Water quality seems fine (my water hardness is always crazy high because of where I live) - NO3 = 25ppm; No2 = 0; ph=7; KH=7ppm; GH=180 Ammonia meter isn't reading anything abnormal. (<0.02 ppm) I've used a round of Melafix (for bacteria) and a round of Pimafix (for fungus), no effect. Tankmates: 4 black skirt tetras, 1 enormous pleco that'll prob live forever, 1 red tailed shark, 1 raphael catfish, 1 (remaining - can't buy these beauties here anymore) golden dojo loach.