He is feeding and acting normally, and as a bit of extra info, I posted a topic a little while ago about strange faeces he was creating (white mucousy film), he is no longer producing these but I still intend to treat him with metronidazole as soon as it arrives in the post.
I have attached some photos, apologies for the poor quality but hopefully they will help with diagnosis, any advice as to the cause of this black mark would be appreciated.
Thanks in advance
Tank Parameters:
pH 7.2
Ammonia 0
Nitrate 20
Nitrite 0
Tank size 106 litres 30"x12"x18" (he will be upgraded to a bigger tank as soon as possible)
Inhabitants: 1x Fahaka
Recent changes: fahaka was moved into his new tank approx 2 weeks ago
The tank has been set up for 3 weeks running on a mature Eheim 2217 filter which has been running on another tank for 2 years
Water changes: 45-60 litres per week in 3-4 installments