Quick answer: This is a regulator accessory from Aqua.
Hoses, adapters and mouthpieces are where a breathing setup gets dialed in. The right hose length and routing can make your rig noticeably more comfortable across a bigger frame.
Offered in 4 options, including Aqua Thin O-Ring #R014 for Tank Valve 5-Pack, Aqua Thin O-Ring #R014 for Tank Valve 2-Pack, Aqua Thin O-Ring #R014 for Tank Valve Single, Aqua Thin O-Ring #R014 for Tank Valve 10-Pack.
Confirm thread size, port type and compatibility with your regulator. Anything that connects to life-support equipment should be installed and checked by a qualified technician. Read Best Scuba Mouthpieces for Comfort and Control. More options: Regulators.