I have 197 Galons tank...i have 1200 liters per hour external canister filter and 2 power filter of 500 liters per hour should i remove power filters....and how often i should change the water.

At the moment with all the filters your tank is turned over about 11 times an hour which is really good.
I would do water changes probably once a week still of about 20%, but you could probably get away with 40% every 2 weeks depending on what is in it.

Thanks racheal......thank you very much well i have 21 goldies at the moment 3 10"huge and two 7" orandas 15 smal 3-4 " goldies