Red/blood Mark

[stu-ton]

I noticed when i came back from holiday the middle of august that my canary yellow goldfish had a red/blood mark underneath it roundabout it flipper , after a few days its dissapeared but last night i noticed its back again, anyone any ideas?

My water test showed everything is ok apart from the general hardeness indicator was showing two out of the 3 indicators changed colour and i dont know if that is bad or not! My water was just changed Saturday

 

[WILDER]

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

Red blood marks beneath the skin is septicemia.