If there is uneaten foodstuffs being disturbed then the answer is YES to overfeeding. The best way to clean the bottom is to invest in a gravel cleaner. It works on the Archimedes principal, very much like your loo cistern. Do you have live plants, and if you do is there algae on them, I would be surprised if you said no. Uneaten fish food will cause high phosphates. Usually live plants can cope with small amounts, but too much will encourage algae. As for your Betta, yes it is quite natural for it to breathe atmospheric air. It has a set of special breathing apparatus in it head to convert atmospheric air to liquified air. Purple for Nitrite. On the scale it shaows about 1 or 2. This is very high. You can lower it by several means-water changes is about the safest, but you can also purchase Nitra zorb, to lower it. Again find the cause and you have the solution.