Bubble in trouble maybe ?

[DEAD_BY_DAWN]

Ive had this little guy for about two weeks now,for the first week everything seemed great ,but for the last week or so it keeps inflating and deflating thoughout the day ,most of the bubbles we keep at the store seem to stay inflated all day and deflate and prouduce feeding tenticles at night,any ideas why mine is acting this way my No3 is a little high 15ppm eveything else including the rest of the corals are doing great