Which is better to relieve slight constipation due to eating too many vegetables, a hot water enema or a fleet enema (sodium bisphosphate)?

Constipation: If you are only mildly constipated you might try using a glycerin suppository instead on an enema.

Before self-treating: Doctors define constipation as stools less frequently than once every 3-4 days. Patients feel they are "constipated" when passing hard or small volume stool, or they feel full ; bloated. Had an x-ray to show you're not blocked? We don't know if your gut is sluggish or your rectum works correctly--these can be tested for (sitz marker study ; anal manometry, respectively). See a GI specialist.