For other people having this problem :
Turns out that it would take a surgery to precisely say for the vet what it is. He did an ultrasonic check and we tried medication for internal infections- which didn't help.
In the end he pulled the water (it was slight yellowish water in the internal "bubble") out with a syringe.
Will have to repeat this now as the only solution every few months. (stuff builds up slowly)
Ringo is relieved to get rid of the liquid and does eat and all. He is 100% fit and healthy besides this internal Bubble.
So a surgery is unneeded suffering and will be avoided.