Acrylic 3-way dosing container holder by @cromag27 of Octo Aquatics. Top-notch quality. Typically use for VOSS water bottles. Much better than the 3D printed hangers by Printed3d.parts that sandwich the hanger and bottle lid between the flange of the RO bulkhead and its nut. Makes it difficult to remove the bottle lid. With the OA the bottles slide right out of the rack.