goldfish going to top of tank

Question
I have two fancy goldfish; lately one and sometimes both go to the top, looking for air I would guess.  I have taking the top of the aquarium off for the meantime, but that is not a great solution.  What would be the reason for this and how do I resolve.  I have a filter; heater; pump; gravel and one fake plant...just incase...

Thanks

Answer
they need more oxygen, your filter probably isn good enough..they need STRONG filters..

add an airstone and change about 50% of the water..but not all at once..like 25% one day and 25% 3 days later..

but some oxygen tablets if you can..they are usually cheap

goos luck!