Golfish hanging near surface of water...though not gasping...?

I have just changed my water - 50% change, normally do this once every two weeks with no problems. Today, the fish are hanging near the surface after the water change, have cleared the bottom of small gravel and replaced with large pebbles (fish getting very big), have also change one of the zeolite compartments. The fish are hanging near the surface though they don't seem to be gasping for air, they have their mouth's closed as if they're smelling something, they don't reach the top of the water, just hanging beneath the surface....any idea what is wrong?