You have a protein film which occurs when there isn't enough surface agitation of the water. If you have an HOB
filter lower the water level enough so that the water flows into the tank like a waterfall. You can also add a bubbler that will cause more agitation of the surface water. To help it along you can lay paper towels flat at one end of the tank and slowly pull them to the other end. This may have to be done several times but it will help remove much of the protein layer. Then do the above to keep it from reforming.