Why are my fish turning yellow?

ok i have a 55 gallon tank with 4 angels, 6 serpae tetras,5 albino cory cats, 4 otocinclus catfish, a small pleco (i know ill have to upgrade when hes bigger) and some stray guppies (4 left, im slowly catching the guppies to move them to another tank...) i do 2 partial water changes a week (15 gallons on monday and thursday) and the last thursday of every month i do 25 gallons which is about 50 %. so the water is good. i have the drop testers and check it a couple hours after every change and the next day if it wasnt good the night before. the tanks been established almost 3 years and i know right now im in a ph spike because of the new fish but why are the white parts on the 2 marble angels turning yellow? is this normal? should i do my water change today or am i ok to wait till tomorrow and stay with the normal schedule? all the fish are still active and acting normal. i just cheched the level and the ammonia NH3/ NH4 are at 0. the ph is a little high but like i said, its spikeing with the new fish. i have aquasafe and prime as my main water treatments along with start zyme, aqua safe and correct ph that i can add to the tank but i dont want to change anything to fast and stress the fish. i added the angels monday after the water change and i did everything right with floating the bag and adding a cup of water. any advice is appreciated. thanks. :)