Thank you all again for your advice and responses. I was in the process of re-housing the Pleco but when I got home from work yesterday found him dead in the tank - he was alive when I left for work yesterday morning. Not too sure what happened there. I'm afraid it was my fault and didn't feed him enough or stressed him out with water changes. He seemed to be much more stressed than my Betta when I would do water changes. My Betta doesn't seem phased by them. I did a 100% water change, rinsed my filter media and put the Betta back in the tank late last night after the filter ran for a few hours.
I do have another question about some of his behavior though. First of all, he's very curious and playful - he follows my finger all over the tank and overall seems pretty happy. However, ever since I bought him (April 17) when someone walks over to his tank he swims erratically darting all over the place. Sometimes when he does this, he will stop and rest on the bottom out of the blue and then start swimming again. It looked yesterday like he was swimming straight up and down a little bit - could this possibly be swim bladder? or is something else going on here? This morning he was darting to the bottom of the tank and bouncing off the gravel a little bit. I've been spending a lot of time with him everyday to monitor his behavior so that I know what's normal and what isn't but I just want to make sure that I am taking the best care of him that I possibly can which is why I'm here. Again, he's my first Betta so if it sounds like I am doing anything wrong, please let me know!! By the way - the white mark on his head went away. I am wondering if the Pleco wasn't terrorizing the poor guy at night. What do you suggest as a good tank mate for a Betta or would you suggest nothing at all? thanks again