My fish seem to be staying at the top of the tank more.?

Hi, i have got 4 goldfish so far in my tank,ive just done a quarter tank change,done my test with my test strip to make sure alls how it should be and it was,but for some reason my fish seem to be staying at the top of the tank and i dont no why.I have increased the oxygen flow around the tank but it doesnt seem to have made any difference.can you help???