Beta has one eye that has bubbled

Question
My daughter got her beta 3 days ago.  It is in a 10 gallon tank which is heated with a filter/pump.  This morning we noticed that one of his eyes is ballooned with an opaque bubble on it... I spoke to the pet store and they suggested there might be a scratch and to treat with salt, but they had no idea how much salt... I'm not sure if this will go away or is a real problem so any suggestions would be great.
thanks

Answer
Hi Lindsay,
Ouch! Sorry to hear! This little betta needs help fast...

It sounds to me like 'pop-eye'--- a condition where fluid buils up behind the eye due to a number of issues.

I feel it would be best to read this article specifically about bettas with 'pop-eye' -
http://nippyfish.net/sick-betta/popeye/

Salt alone generally will not cure it. Be sure to only use aquarium salt or pure rock salt. Never table salt as it can be harmful due to additives. Make sure to step up on the water changes anytime illness like popeye strikes.. 30% every other day would be ideal. Make sure to always use a water conditioner and try to match the temps as close as possible between the new water and the old original water.

-Sometimes popeye will reverse itself with good care (water changes) and time.

Best of luck!! I hope this helps!
Susan~