My betta Aoi Akai is weak, she lays on the ground all day and will only swim to the top when she needs air or eats, she acts normal, she eats all the food I give her and go to the bathroom regularly.
I've had a few other bettas do this I don't know it this is a diseases or if it's because they get lazy and loose musile, it dosn't seem to be catching because I've had her in a 10 gal. split in to 3 and the other bettas were fine (sadly the other two died of ick).
The tank that I had her in when she got this was a 10 gal. split in 4 the deviders was plastic mesh that they couldent see through (which leads me to think it's mucile loss). The tank was heated to around 80 deg the water was good so there was no reason for this to happen.