Yes, an arched back is abnormal... I believe Callistra said that. I don't know exactly what that means, but the only Betta I have ever seen with an arched back like that had extremely bad fin rot and was missing most of his fins, so I'm assuming that Betta was swimming strangely because he had almost no fins...
And yes, that is way too long to leave the lights on. You only want them on for 8-12 hours a day. I personally leave mine on for ten and I have a light timer so they turn on and off on their own if I'm not around at a normal time. They need to turn on and off at around the same time each day to establish a good day and night cycle. Also, bettas don't have eyelids so they can't really sleep with the lights on. Not enough sleep can cause stress. Your tank is likely growing algae because of the light schedule...
I know some people aren't around a lot to be able to turn the lights on and off at a consistent schedule. I purchased a light timer for my four tanks with two plug ins that was $8 from Menards, and I got one with only one plug in for my brothers single tank for $4.
If you are cleaning it with conditioned tap water you may not have hurt your cycle too much, but cleaning it with unconditioned tap water would kill your good bacteria and cause an ammonia spike as they died off.. Partial water changes with a siphon are all you will need for a filtered tank, and the cycle will eventually complete itself as long as there is a fish in it..
Edit: btw, has he improved after the water change color wise? When was the last water change? Try leaving the lights off longer and see if he improves. When I put my fish to bed but people are still awake with the room lights on, I cover the front of the tank to keep light out to help them sleep. This may be stress from both not changing the water enough and not getting enough sleep.
I would do two 50%s a week from now on since we don't know if your tank is cycled. Do you have a test kit? (50%s should not take you as long as 100%s, I'm able to do mine in less than 10 minutes). You may want to try different foods. How is he acting, now?
Last edited by AyalaCookiejar; 01-21-2013 at 12:50 PM.