i am confused.... there is a compression fitting that fits on bulkheads? I have never seen any fitting that fits on the thread for the nut of a bulk head before.I may be still a little confused but, if I understand you correctly...
I would turn the pumps off and let the water drain into the sump. Then I would remove the water in the overflow box. It looks like the intake pipes are using a compression fitting. I would remove all three PVC intake pipes by unscrewing the fittings.
At this point you should have a dry overflow box and only the bulkheads and the compression fittings coming out of the overflow box.
You might want to remove the bulkheads and make sure the gaskets (and overflow box) are perfectly clean. Then you can tighten them, hand tight + 1/4 turn. You have to be careful not to over tighten but they should not be spinning when you try to tighten the other fittings.
You can watch the video at the link below on how to properly tighten the bulkheads.
How To Select and Properly Install Aquarium Bulkheads
Bulkheads allow you to transfer water from one side of an aquarium to the other and allow for traditional PVC plumbing to be connected. In most aquariums, bulkheads are used for drain and return pipes on your display tank but can be used for many other things too. Some tanks come pre-drilled for...www.bulkreefsupply.com
All of this is really pretty straight forward and will give you piece of mind you will not have a leak in the future.
Hope this helps.
Steve