Lost my tang and a clown? no idea why

[Dyspnea]

I Think this was an oxygen issue, like the others have touched on. Tangs love oxygenated water, which sounds like there wasn't alot to begin with, since the aquarium is only 30 gallons and only water movement is from a marineland filter. Tangs need alot of space 30 gallons wouldn't have cut it and most tangs need at least a four foot long tank to swim in.