Parabens are a group of chemicals that are used as preservatives. The most common ones in personal care products are methylparaben, ethylparaben, propylparaben and butylparaben. Of these, methylparaben is the smallest molecule and butylparaben is the largest. Often, more than one paraben is used in the same product to achieve the desired effect.
Parabens have become well known as endocrine disruptors. Tests have shown that some parabens can mimic estrogen. The larger parabens seem to mimic estrogen more strongly, which makes them more toxic.
Because of this, ethylparaben, propylparaben and butylparaben are all toxic to aquatic life. Methylparaben, being smaller, does not strongly mimic estrogen and is less toxic. It is harmful to aquatic life. Given it is also readily biodegradable, methylparaben is not too bad a choice as a preservative.