Complex sentence: The Adonis tetra can reach 2.1 centimetres long so is a very small fish, even smaller than the neon tetra, and is one of the smallest fish normally kept in aquariums.
Split into simpler sentences: The Adonis Tetra can reach 2.1 centimetres long so it is a very small fish, even smaller than the Neon Tetra. In fact it is one of the smallest fish normally kept in aquariums.