Black patch on caudal fin

[Allison_ingram]

Hi guys, after some advice for my male apisto Cac.

He has developed a black swollen patch on his caudal fin.

He's eating fine and swimming about happily although I think he's gotten a bit skinny

PH 6.8
Ammonia 0.25
Nitrites 0.25
Nitrates 40

I'm going through a mini cycle, doing frequent water changes and double dosing with prime

Established tank, fishless cycle in June 2016, 3x apisto introduced in August 2016

Tank mates are 8x harlequin rasbora and 4x corydoras

125l tank with sand substrate

I've attached pictures and would be grateful for any input x

 

[DutchMuch]

not answering the question, but corys need at least a group of six.

 

[Allison_ingram]

Yeah I know, I'm hoping to get another 4 once I've got my issues sorted x

Any idea about the black patch?

Thanks

 

[DutchMuch]

Yeah I know, I'm hoping to get another 4 once I've got my issues sorted x

Any idea about the black patch?

Thanks

is this actually a patch, or is it flat against the fin? could just be an abnormal fin growth but I don't want to make any assumptions right yet.

 

[Allison_ingram]

It's actually like a lump when you see him face on. He's just not himself tonight, although swimming about happily and eating, he's not displaying to the female anymore.