Melev makes an awesome top-down box. I’m using the first-generation one, it gets around glare because the bottom of the box (the part the lens shoots through) is submerged below the surface, where the glare is.
It also has a lip on it, which hooks onto the tank. That helps stabilize for the shot, as does the upward pressure of the water trying to push the box out of the water. The resistance tends to help stabilize.
Finally, there’s always ramping up your shutter speed.
