what's wrong with my guppy's tail?

[thisisyesterday]

one of my guppies developed a small streak in its tail, quite dark in colour. it has got bigger and bigger

is this normal or a problem??

dh is a fishy man (he works in an aquatics shop) if its getting larger and is a black line its probably just colouration of the tail as they are reaching maturity (obviously if its a young ish one)
Female-ditto when reaches maturity.
As long as its body isnt swelling up bigger at the same time or have red lines like veins coming off it. If it has that means its a bacterial infection and needs sorting.

Oh Jeez! That'll teach me to wear my glasses at all times!

I read your OP this morning, thisisyesterday... but I thought you'd asked "What's wrong with my puppy's tail"!

No wonder that I couldn't make sense of your question!

thanks, it's just a dark line that's getting fatter atm, no red lines and no body swelling