Turn out your lights completly
DO NOT change your filter or drain the tank completly, it doesnt matter that your using your old gravel, the bacteria colonise mainly in the filter, if you change all your water, the difference in qualities will no doubt wipe out your filter. as i said turn out your lights, use a product call ACU-CLEAR, which is a floculating agent and works by binding water bourne particulate matter together. and simply wait. this will be the best thing for your fish as you will be reducing stress levels. if your water quality was fine then the fish will be fine. there is a difference between water clarity and water quality. water clarity wont kill your fish, water quality will!
good luck