Betta sick and I don't know why? Help!

Mar 30, 2018
I have had my betta for a few months now and over the past 1 or 2 days he has just sat on the bottom of the tank and hasn't eaten, when i go to see if hes alive (by brushing water around him) he darts around very quickly and returns to the same spot on the ground. I looked online and did the obvious research so I purchased a full test kit for the water. My temperature fine, my pH fine, my Ammonia fine, my Nitrite fine and my Nitrate fine so i'm really lost. I do weekly water changes his tank is clean from algae, he has live plants and is in a 10L tank I feed him omega one pellets, usually 3 in the morning and night, around 8'o'clock each time. I am just hoping for an insight to help my poor boy, but I have looked all over google and forums but there has been no answer to my problem. Thank you for any and all answers