Type of bulkhead (wet side + dry side) - threaded + threaded; threaded + slip or slip

[byee]

My stand for my 45g cube is finally finished and I'll need to water test my tank for leaks.

There are may choices for bulk heads (wet + dry side) threaded & threaded, threaded & slip and slip & slip.

My cube came with threaded & threaded. On the wet side, I wasn't able to tighten the 3/4" threaded tube into the bulkhead connector. However, I had no problems with the 1" threaded overflows. In fact, I had a couple of rubber O-rings which I installed in between the threaded piece and the bulkhead making it water tight.

I'm worried that the threaded + threaded bulkheads will eventually develop a slow leak over time.

I'm interested in everyone's opinion for the different types.

Thank you!