I have an axolotl that in the past 24hrs has developed black toes/feet and a black mark under his mouth. I have had my axolotl for about a month. I have been doing water changes every week. The ph is at 7 to 7.2. I have a filter that hangs on the side of the tank, which I have turned down low this morning, (I thought maybe the flow may of been to high and he was stressing). I noticed one of his gills where up (from what I have read this is a sign of stress) so I am hoping by turning down the flow it reduces his stress. I have been feeding him bloodworms and pellets (he is not fused on pellets). Is there anything that I am doing wrong?
My main concern is that he has developed these black toes/feet is this normal or treatable?
Can anyone possible advice me on what to do?

My main concern is that he has developed these black toes/feet is this normal or treatable?
Can anyone possible advice me on what to do?