The sports drinks tend to be somewhat hypotonic, but are probably a better choice than plan water, and the low calorie versions are preferable. The bowel Flora are not accustomed to seeing much sugar, so have a 'field' day, and produce lots gas by fermenting the sugars.
If you really want an isotonic enema, use something like GoLytely, or Tri-Lyte (PEG/Electrolyte solutions used for Bowel Cleansing prior to Colonoscopy). While these products require a pre-scription in the USA, they don't in most of the rest of the world. Alternatively the chemicals used in these products are not exactly hazardous, or difficult to obtain. Making your own is certainly possible (I have done it). The ingredients and amounts are not a secret. It is on the label, and many of these products have web sites that provide that information as well. Just make sure you use products that are safe for human consumption. Baking soda from the grocery store shelf, Salt substitue (KCl) and non-iodized table salt. Miralax, Dulcolax Balance or similair PEG3350 based laxative for the PEG. Ingredients plus a good scale is all you need.
I often refer to these PEG based products as 'oral enemas', because they are designed to go in one end, and will come out the other end pretty much unchanged. The PEG3350 makes it almost impossible to absorb the water, and the net electrolytes gain/loss is near zero.
As a practical matter however, it is difficult to produce significant electrolyte anomolies with tap water or distilled water enemas absent significant disease.