Is it okay to use warm water instead of a fleet enema (sodium bisphosphate)?

Enema. Tap water enemas are fine. However, if you are having problems with constipation at age 53, you should see your physician for further workup.
Yes. It really depends why you are doing an enema. If you have been advised to do a fleets enema in preparation for a procedure, it would be best to follow that recommendation. If you are taking a daily or periodic enema to aid bowel function, then tap water may be adequate. In theory, warm water relaxes the bowel, so a warm water enema may be less effective than a room or tap temperature enema.