I grow beard algae for my gobies and had a 20L set up for the algae. I did not scrub the sides, simply rolling a plastic plant to collect the algae for placement in the goby tank. About a month ago I moved the algae to a storage container and reclaimed the tank for a male guppy display tank. Last weekend I put a black background on it and moved the light to the front of the tank to highlight the guppy colors. It works well and looks quite dramatic except that some of the front glass looks cloudy as if it had a slight film on it. It's definitely on the inside and I also noticed that the magnet cleaner shimmies when I try to clear it away. Yet, there is absolutely no visible algae, even when scrutinized with my nose on the glass. In addition, it is not an even coating, more like clouds on a summer day with oddly shaped clear areas.

When I reclaimed the tank I used netting and the magnet to brush the sides. Is it possible that the algae permanently effected my tank with tiny pits or something? Any suggestion on how to remove this?

Thanks for your thoughts,