How to increase Dissolved Oxygen in a tank

Hello so guys i was wondering what are the best ways to increase oxygen in a tank as i do not plan on acquiring a chiller so i want to know ways i can increase oxygen so my fish don't die at night due to lack of oxygen. Would this do for a 55 gallon tank, i do not have a Sump as i am a student on a tight budget, also tank is a FOWLR

-Skimmer
-2 airstone
-HOB Emperor 400 filter
-3 Wave maker/pumps

Would this be enough wht else would u guys recommend so my fish won't suffer from lack of oxygen

Thanks