Blue Tang had a HUGE eye
 

but the other is normal....what do I do?

Sounds like a case of pop-eye. Pop-eye is mainly caused by improper water condition/environment.Can you please list your exact water levels for me?

I would start off by telling me a little more about your tank as well as stocking list and ect. It could just be a cause of poor water quality which can be solved rather easily. There are treatments available out there for pop-eye but more info is needed to determine the problem.

125 6ft
5 Barlet Anthias
1 Sohal
1 Scorpas
1 Blue Tang
1 Tomatoe Clown
1 Royal Gramma

Temp 77-79 (depending on time of day)
3 weeks ago I went barebottom so as of late there have been 5-10 gallon water changes 3 times a week (siphoning)

Whats your water levels?

Unilateral pop-eye is usually the result of trauma/injury.

Tangs are very active and hyper. I refer to them as the dart/dash fish or ADHD fish. They are notorious for getting scared or startled and heading for cover.

Secondary infections can occur if water parameters are not pristine and/or there is chronic or acute stress.

Fish with eye injuries may be the subject of additional trauma due to other fish trying to pick on the eye or "taste it" or the tang trying to get away from them.