An apparatus for processing fluid includes a vessel provided with a sidewall including a passage and at least partially defining an interior compartment. A drain tube positioned in the interior compartment may drain fluid through the passage in the sidewall. The drain tube may be angled, and arranged such that the end in the interior compartment may be adapted for movement as the result of manipulation of a structure, such as a coupler, external to the interior compartment. Accordingly, the drain tube may be positioned along the sidewall at a reference point for movement between a first position for draining fluid above the reference point and a second position for draining fluid below the reference point. An indexing feature may also be provided for incrementally adjusting the position of the drain tube, along with an associated tool to facilitate the adjustment. Related methods are also disclosed.