[jerryclunsford]

Really not enough information to figure anything.
Low oxygen will kill larger life before the corals.
Most toxins will kill the corals too.

 

[jerryclunsford]

Was the water cloudy at all.
Was thinking that if the coral food was bad might have caused bacterial bloom using the O2.

 

[jerryclunsford]

You don’t really want the power head to make bubbles. They pop and spray salt. You do want good surface agitation for better gas exchange.
Do you have a sump or protein skimmer?

 

[jerryclunsford]

Normally your water should be crystal clear.
A bacterial bloom will cloud the water.

 

[jerryclunsford]

You don’t have to have a sump I ran a 36 gallon for years without one. A protein skimmer is also not necessary but it will help keep water quality up and will help oxygenate the water.
I would increase surface agitation and consider getting a skimmer at some point. I just got a skimmer for my 65 gallon tank. It has run around 5 years without one.

 

[jerryclunsford]

A better power head would help. You can’t really direct that one at the surface to get agitation. It doesn’t look like it would produce much flow either. I use a gyre generator produces a lot of flow. Your flow should be at least 10 times the water volume most people recommend more. (Much more)
As for the skimmer reef octopus makes some good ones as does I think it is called Life Reef. Hack has one of theirs a lot depends on your budget.

 

[jerryclunsford]

I wouldn’t. You may be able to adjust the canister return for more surface agitation.

 

[jerryclunsford]

You can wait a while on the skimmer and save up some.

 

[jerryclunsford]

Remember that low O2 is only a theory we don’t have enough to work on.
Your tank will benefit from better gas exchange though.