Hose filter to remove chlorine? Help

I just added a auto water pond filler. Is ther a Chlorine and heavy
metal garden hose filter? I sometimes put my backflush on to clean my
filter. I let it run in the garden then pour garden hose water in. so
far no problems. Bur if I ever do a 10-20% water change (300 gallon
pond) I dont want anything bad to happen. How much of a water change can
I do without adding declor? I can no longer lift buckets of water that i
declor to pour in the pond. Is it safe to add some tap water without
hurting the fish? I was hoping I could just backflush about 30 gallons a
week the the auto filler would fill it. Please let me knowThank again