guppy has white streak on his top fin

i have a 10 us gallon tank with 4 lyretail guppies and a ottocinclus. my biggest guppy has recently gotten a white streak on his top fin. he has always been bigger than the others and had a larger top fin. They're all malds and cane from the same pet store. Other than the white streak on his fin he hasn't had any other symptoms and the others are perfectly fine.
water parameters
ph - 7.6
high range ph- 7.4
ammonia - 0.25
nitrite -0
nitrate-0

What white streak?

CryptFan said:

What white streak?

Click to expand...

Unfortunately it’s hard to see in the pictures but these were the best I could get. The stream is in the middle of his top fin.