What is the temp?
Did you age the water, 24hrs with aeration before doing a wc? Often the ph is not stable when first getting water from the tap. Changing ph will stress the fish.
Also, such a lg wc can stress the fish. It is better IMO to do 50% wc at a time.
The filter foam should be cleaned in used aquarium water to preserve the good bacteria.
WHat is ammonia, nitrite?
What size tank? How long have you had the fish? How big are the fish?
hth