Is my betta fish dying?

I got Mr. Grumpy Gills for my birthday a few days ago. the water was getting gross so i changed the whole fishbowl (the man at the pet store told me to only do half the bowl but the water was really gross so i changed the whole bowl) i did half spring half tap water at the same temp as the old water and ever since i changed it, Mr Grumpy Gills has been unusually mopey. he floats on the top completely motionless or on the bottom almost laying down on his side. he lays on his fake plant like its a couch now, and its kinda cute but now im scared he's dying :((( this is what happened to my last betta. he got mopey after i changed water and died the next morning :( is he dying or just sleeping?