Most of us experienced fish keepers change water weekly, 30% to even 90%. pH is very much less important that ammonia, nitrIte & nitrAte especially in a tank that is likely over stocked. Take a water sample to a local shop & don't settle for "it's fine"; we want actual numbers!
So, you didn't say what size tank or size & number of each fish. We need to know.
What your seeing could be from poor water quality. Your pleco looks like it may be thin too. What & how much do you feed? Don't expect it to just eat algae, most are omnivores (meat & plant eaters).