Lift capacities for recreational diving are largely a comfort issue - higher lift equals a higher head position above the water on surface, which also equals wider wings, which require more movement underwater to shift the air precisely.
The minimal lift of a bladder of course should depend on what kind of tank you're diving, steel or aluminium. 18lbs, you'll definitely be struggling to keep afloat when using a 15L steel tank for example. Your lead weights also play a part towards sinking you on the surface, so don't forget to factor those in.
Generally I would say at least 25lbs if you're diving in wetsuits, 30+lbs if boardshorts + rashguard or for more flexibility. Extra lift is always better than keeping everything overly small and compact. That additional lift can come in handy, such as long waits on surface or choppy seas, or even when you need to perform a rescue.
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