<p>Fig. 2. A serrated bread knife works well for cutting large plastic bottles. Cut your bottle at the&nbsp;so the lower aquarium portion will have a bit of curvature at the top.</p>

Fig. 2. A serrated bread knife works well for cutting large plastic bottles. Cut your bottle at the so the lower aquarium portion will have a bit of curvature at the top.

Image by Kanesa Duncan Seraphin