Clownfish- not eating swimming erratically

[Regatta]

I have had my clown for about a year and a half. Total system volume is 12 gallons. Only other livestock is a goby CUC and corals.

Yesterday my clown started exhibiting some very worrisome symptoms. Swimming erratically as if having seizures then "stalling out". Her spine would curl over she would stop breathing before sinking to the bottom and then springing to life again. This was about every minute all day yesterday and all night. 

I tested water parameters:
Ammonia 0
Nitrite 0
Nitrate 10

Nothing else in the tank is behaving strangely, and everyone else appears healthy. I did a 40% water change after noticing her symptoms.

Today she is still exhibiting the same behaviors, only now she will float to the top when "stalling out" instead of the bottom. No sign of parasites or infection. I tried feeding peas in case of a digestive/swim bladder issue but she will not eat. Not sure if it is swim bladder related because at times she darts around the tank quite competently. I am worried for her, any ideas on how to treat?

[Regatta]

Video of behavior 

 

[Clown79]

I have never seen that occur before. 

Any other symptoms? 

 

https://www.google.ca/amp/saltwateraquariumblog.com/swim-bladder-disease/amp/

 

Here is a good article on swim bladder.

 

Try posting this with emergency in the title in the general  forum. You may get more hits.