i've been told numerous times by different fish shops and friends that its not
Are you sure of those stats? Its very very rare to have zero nitrates.
Sorry I can't believe you have a ammonia and nitrite reading of 0.
The tank is severly overtstocked and you are only doing water changes once a fortnight.
Plus the shark is gasping a sign of bad water quality.
Immediate water change and I would rehome some of the fish.
The sharks sounds like its dying of ammonia poisoning..