Need help for my oscar fish almost 5''inches.ASAP/Please.I lost 2 of my albinos in 2weeks?

i have had 6 oscars 4 albinos and two tigers all are almost 5/6inches.!lately they have not been eating regularly amost for days.i went to my pet shop and he recommended me to feed them live gold fishes and check the water leavel it all seemes ok than.they were all exicted feeding on live fishes.it was just once i tried and it worked in a weeks time one of my albinos oscar was just goin bizzar and dropping down,grasping for oxegen i lost him.same in the following week i lost another.i regular change the filter weekly and 50% of the water.and now they have stop feeding on pellets-tetra brand.please help im worried doesnt make me feell ok..as im loosing them it almost 3years they r with me. My Email id- lean_clement@yahoo.com.