this is how you fix this problem: remove the rear wheel. count how many links you have in your chain and then divide that number 1.56 and then add the number of spokes you have in the rear wheel to the total. Use this number to reach the proper chain tension -- this is called your "fixie gain ratio". See Sheldon Brown's site -- he has a chart that lists various fixie gain ratios and their correspondent measurements of how far back an axle should sit in the rear drop outs. When you put the rear wheel back on, be sure to slightly tilt the rear wheel toward the crank. This will make your pedaling much more efficient (which is also known as "go faster").