Unlikely to be an issue with the salinity
per se, so I wouldn't worry too much about that. Schaefer reckons T. microlepis will tolerate (even if it doesn't like) ~50% seawater, and you're well below that.
Now, there are three things that spring to mind.
The first is super-saturation of gas in the water. This is normally a problem in marine tanks rather than freshwater tanks. As I understand it (and I could be wrong, since I don't really follow the marine side of things) the salt effectively "squeezes" out the dissolved gases in the water, making it easier for gas bubbles to accumulate in the fish's tissues. This doesn't normally happen, but if the water is very vigourously aerated, it can happen. Goes away by itself eventually, provided the conditions in the aquarium are good.
The second idea is mechanical damage. The eyes of fish are easily damaged; by fighting, by being netted, or by bumping/leaping into solid objects. Treating for external secondary infections (i.e., finrot/fungus) would be a good idea, but the salty water should help keep things pretty clean anyway. What tends to happen is the transparent skin over the eye gradually turns cloudy, a bit like a combination of conjunctivitis and eye cataracts in humans. Advanced cases don't really seem to be treatable (as far as I know) though they may improve over time.
The final idea is pop-eye. But that's not really a bubble over the eye but behind it, pushing the eyeball outwards.
Eye flukes are something you might consider. They look like scratches or cloudy patches on the eye. You can't really treat them, but luckily once they complete their life cycle, they can't infect the fish again (or any other fish) because the intermediate hosts they need are missing from the aquarium. Light infections cause no serious harm in the long term.
Cheers, Neale
I have 4 Archers (Toxotes Microlepis). They are all about 4/5 inch.
I have noticed that the smallest one got what looks like a cloudy bubble over his eyes. You really have to take a close look to see it, he is eating ok, and my other Archers and fish are ok. The only thing I can think of is that the salinity is too high. I know they should be in 1.005. they are in about 1.008 after lowering my salinity.
Any Ideas what it could be?
/www.fishjunkies.com/Diseases/popeye.php
