Good way to stop electrolyte tabs from turning into powder?
I've got a few individually wrapped tablets (and equivalent powders) that came as samples.  I never use them when starting from home instead saving them for carrying with me.  This works for long hot days, but on a multi-day trip would soon run out, and the packaging is wasteful.

The tubes protect the tablets from crushing, though they add bulk compared to decanting into a small resealable bag and packing that carefully into your luggage.  

Assuming you want to carry on using the original tube, you need to fill the empty space.  You could use something as simple as paper or cotton wool, or even the wrappers from cereal bars, screwed up.  I wouldn't use gel wrappers as they'll make the tablets damp. If touring with the ability to make hot drinks you could even use tea bags as padding. Some of the tubes of tablets have a sort of plastic spring that takes up the space when only a few have gone.  This seems effective for as long as it lasts, so I suggest filling the space under the tablets.  For that you could even use a few sweets (making assumptions about what else you carry).  Under the tablets, enclosed in a bag, would be a good place to carry some emergency cash, making good use of the space.