My 30-gallon tank used to have the same problem as yours and my fish died because I found out the reason too late. After that, I bought a powerhead for my tank to control the water flow and remain Oxy stable in my tank.
I think gravel is not a bad idea. I put Flourite black gravel in my tank and it looks very nice. I did rinse it first by poking holes in the bottom of the bag and placing it in a bucket. After reading other reviews I decided to bake it in the sun for 3 hours. Hardly any clouding of the water.