hey pine,, standpipes, bell siphons and loop siphons are usually used, what is your gb made of?
there are bulkhead fittings or uniseal,
oh, and don't forget silicone (aquarium safe of course!)

i threaded pvc fittings through plastic barrels (drill the hole and use a metal nipple heated up to thread the hole) with a lock nut on one side and silicone around the threads