kosamchetoo :
But the bottom fan will decrease GPU temperature right? Because my GPU is 70 degrees C under load and i think thats a bit high.
70 degrees is not unusually high, i would start to worry if it started to get up to 80 to 85 degrees.
A fan on the bottom would help a little with GPU temps a little but only by a few degrees, maybe look at your GPU fan curve, changing this should help with your GPU temps but incresase your fan noise.
Honestly if your GPU is reaching 70 Degrees under max load, e.g. stress tests. Then you have nothing to worry about, and that's without knowing your card type e.g. my GTX 970 reaches a max of 69 degrees.
Hope that answers your questions.