I would not use glass. I would go for acrylic or polycarbonate. They both
have better impact resistance than glass, that way you are less likely to
have "blow out" and lose all you fish.

Acrylic has about 4 times the strength of glass. Polycarbonate is reputed to
be about 250 times.

If I were doing a window in a pond I would go for .25-inch material in
either case.

Only problem will be expense.