pyrex is extremeley strong, it has physical properties unlike normal glass.
in fact, the secret to making pyrex has been just that for quite a long time and still is, property of Corning USA and licensed to one other manufacturer in Germany I think.
Pyrex has much more tensile strength than normal glass. Tempered glasses can have varying tensile strengths and hardnesses. I know that pyrex has Boron in it (boro-silicate), which is maybe a heavy metal? I'm no chemist. Perhaps its the boron that makes it unsuitable for fish and plants, but its still suitable for cooking so maybe there's a more complex reaction.
"Boron is an essential plant nutrient, although soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron in the soil is greater than 1.8 ppm. When Boron in the soil exceeds 2.0 ppm, few plants will perform well. Plants sensitive to boron in the soil may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear. As an ultratrace mineral, boron is necessary for the optimal health of animals, though its physiological role in animals is poorly understood."
however, there is a kind of glass which is nowadays referred to as quartz, which is horribly misleading marketing. It maybe be made from melted down Quartz, but once quartz melts it's just sand or glass, good ol' silicon dioxide. However, as the name truly implies, quartz can easily be nigh on totally pure SiO2, and when glass is made from pure SiO2 it melts at an extremeley high temperature, is perfectly clear (blow your mind), and DENSE. DENSE AND HARD, HARD AND STRONG.
this kind of glass is so expensive to produce, however, it is rarely used for nonscientific purposes. It'd make a beautiful, heavy, expensive tank though.