Um yehhhhhhh.........you can change the gravel but here's the thing. Take all your decor out the aquarium.before you start. First things first make sure you wash the sand first to get that loose debris out. Also it will minimize cloudiness. Next I would drop the water level to maybe 50%. Pour sand in SLOWLY. If you don't your aquarium will be very blurry and hazy. The best idea woulda been to add the sand before the water when you set your aquarium up but it too late now. When the sand is in slowly move your hand to spread the sand throughout the tank. Again, moving slowly will help minimize the cloudiness. Dont just dump a huge bag of sand in your tank ,do it on intervals to minimize the stress level of your fishies.After this is done then you want to add your rocks and other decor.