Need help.

One of my german blue rams has not been eating much or active latley and now i havent seen him eat in a few days and he is very skinny. He has also turned dark in color and sits on the bottom and doesnt move around lots. Any ideas what to do? I tested the water and the ph was low around 6.2-6.4. I did a few water changes and i dont know what it could be. the rest of my fish are healthy and active. temp is at 78-80. thanks

sounds like it could be internal parasites. Have you ever seen white stringy poo instead of normal looking poo?