Betta on the Bottom of the tank

Question
I'm having problems with my betta, Sam.  We got him from our neighbors about 2 months ago in his original tank.  We recently (about a month ago) changed out his gravel and added a plant to his 1 1/3 gallon tank. Tank has a bubble filter and we change about 1/4 to 1/3 of the water out weekly.

About 4 days ago, Sam started acting very sluggish and laying on the bottom of the tank (sometimes so drastically that he's almost on his side).  He will occasionally dart around the tank in short quick bursts, but has eaten very little (last time he ate was roughly 2 days ago).  He has no marks on his body and no unusual swelling.  Looks healthy except for the lethargy.

Today when we changed the water, we added a slightly warmer amount of water to the tank, to see if that might rouse him.  He's darted around a bit more as a result, but nothing else. Thank you for any help!

Answer
Rebecca,

Sorry to hear Sam is not well. Let's get him back to 100%. The first thing I would do is remove the filter. Betta's don't like them nor do they need them. Plus, the tank is too small for a filter, not to mention a Betta. You do, however need a heater. Betta's are tropical fish and need their water to be at 82 degrees nothing less. If their water gets too cold they do exactly what he is doing now. They don't swim nor will they eat. Also, please, as soon as you can get him a larger tank. With the gravel and the heater, it is way too small. You can get a very inexpensive 5 gallon with everything you need at Walmart. Just don't use the filter. 25% water changes once are week are great. Don't forget to add a water conditioner to the water being added. Make sure it removes ammonia, nitrates and nitrites. Once you do this Sam will be much happier and will live a lot longer.