In the fluval X04 series canister filter,if the water inlet is submerge, the only air intake is the priming device (if all air pocket in the canister is release). Due to wear and tear the priming "stick" will loose and result air been suck via the priming stick's hole. Apply some Vaseline around the priming "stick" will help.
The other reason the bubble form is due to the high pressure during the water intake and the low pressure in the canister outlet. As result, air slowly accumulate in a pocket and is release every 1-2 hours when the air pocket reach certain volume.
As there is very little you can do on the dissolve air in the water, just make sure there is as little as possible air pocket trap in from canister to water outlet.