Complex sentence: Aboleths are fish-like amphibians, twenty feet long and weighing about 6,500 pounds; they continue to grow as they age, however, and some fantastically ancient specimens reach much longer lengths.
Split into simpler sentences: Aboleths are fish-like amphibians, twenty feet long and weighing about 6,500 pounds. However, they continue to grow as they age, and some fantastically ancient specimens reach much longer lengths.