My fish tank is cloudy and it has been up for sometime.?

OK. My fish tank is a 15g and it has been up since the 5th of June and I just put in 3 young rasboras and 3 young glow-light tetras have just been added today on the 26th of June along with a octoclinous and a corydora. The tank now seem cloudy and I think it is low oxygen. What is the reason for this. Also I have a small plant inside the tank. What is cheapest way to get oxegenn in the tank. Ideally I don't whant to bye anything.