Originally Posted by DJAinKY
Admittedly, I was hazily recalling what I remembered from reading about it when that happened. This is what I was referring to:
"Pop eye can also be caused by gas bubble disease as a result of oxygen super saturation (excess levels) of the water with the gas, nitrogen. Super saturation occurs whenever the pressure of a gas in the water is higher than the pressure of the same gas in the surrounding atmosphere, whereby the difference in gas pressures causes the gas to get pulled too quickly out of the fish's bloodstream, leaving behind gas bubbles. The other symptoms of this are the appearance of bubbles under the fish's skin. It's caused by excess oxygen in the water, particularly from filters that blow air directly from outside to inside the tank, and from pressurized tap water that did not get mixed."
Same solution, either way. IIRC, the tank in question is filtered and the bioload doesn't seem excessive, so I still think 70% is too much of a water change.
Of course, this could actually be some kind of infection. But if it's happening around the water changes, I don't think it can be discounted.
That is kind of why I gave the example of teleporting to Everest from New York. Since betta tanks are open to the air they don't build up pressure on the inside like tanks that are completely closed off and filled to the top with water. I have seen what you read about happen in a Petco tank that was completely closed and full of water. They should have moved the fish because there was so much excess air in the water that it was turning the skin under their eyes into ballons and some fish had already lost one eye or had both eyes buldging.
So you aren't wrong because gas defussion can happen in tanks that don't have a cross breeze. You happen to be wrong about this perticular based on the picture the OP posted. The pressure on the outside of the tank would have to decrease of a betta to get popeye that way. So that's why I gave the example of teleporting from New York to the top of mount Everest while holding a betta tank. Even the blood inside the human would have trouble with gas defussion in that example.
But I do agree that 70% pwc is a bit of over kill. I wouldn't do much more than 25-50% water changes a week on a cycled tank.