Help! Pleco with spits

[Cindaloo]

I’ve had this pleco about 15 years and this morning I noticed these patches on him! What do I do?!?!

 

[Colin_T]

check the water for ammonia, nitrite nitrate and pH.

do a big water change and gravel clean the substrate.

clean the filter if it hasn't been done in the last 2 weeks.

if it hasn't changed after that it's probably just colouration