I've never understood how people make fish constipated.

Bettas seem to be prone to it. I've never had any fish except bettas
become consitpated. Overfeeding is probably the major cause. Most of
those pellets have only between 6% and 10% moisture and can absorb a
lot of water. The fish eats a few pellets and they swell 2x to 3x their
size. The fish appears very bloated and may not "go" for 3 or 4 days.
Once it passes the fish appears normal again.