Actually, this (that is, being able to give a sableye rock candy in lieu of actual rocks) is a common misconception, but the truth is that sugar is not the best thing for a sableye’s health. It doesn’t digest it like we do, and it can build up in its digestive system, leading to certain health problems.
Instead, consider offering cheap crystals such as polished quartz, which you may be able to find in bulk for less than $10 online. Alternatively, glass (so long as you’ve sanded the edges) and ceramic (same) can work just as well, although they’re not quite as nutritional as tumbled crystals.