Since you started with a full tank of air it may take time to bleed out. From what I've read on the forum some filters have an air bleed valve at the top, others, like my Hayward, seem to have an internal mechanism for bleeding air. If the bubbles don't go away then the air has to be getting sucked in on the suction side of the pump somewhere. Changing the sand, no matter how you screw up, should cause an ongoing bubble problem.
Let us know if the situation has improved.
Al
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