Seamus Durrack recommends Hack Up Liquid Electrolytes.

I've been getting barrels of electrolytes off Alex for 3 years now, and wouldn't change.

It's really important to be able to rehydrate my horses at any particular time and the fact that this is a liquid  enables me to syringe it down at any point. The horses eat up their feed well when it's in too. Works well and wouldn't swap it for anything else.