I had the same issue, he would deflate, when irritated or pooping etc.... then would reapear, all normal again, they do this sometimes, mine did, infortunately, i was cleaning my tank and he floated loose and got sucked into the pump, ouchhhhhhhhh, so i guess im a murderer, i learned a leason there, when they dissapear they can be very hard to find again, but it usually is caused by stress or something like that, if he constantly does it, you might want to make sure hes not being pestered !!! hope this hellps !!