is there any way to get oxegen into my fish tank?

[stephen G]

my platy has been breathing very hard and I want to know if I can get oxengen in my fish tank. i do have a filter and tere is 4 fish in the tank. shes breathing hard and that might be that theres not a lot of oxegen.

ik you can get a air pump. somthing that I dont have to buy @ a store..

any ideas.
its a 5 gallon tank and has 4 small fish in it. 2 are platys, 2 are rasbora. i do water changes every week.