There have been a lot of posts on using safe substrates.
Quite frankly, the safest substrate to prevent impactions is either none at all, or a single large bare rock. This definitely can't be ingested by a gecko. <g>
However, is safety the only issue being addressed? I submit that psychology should also be considered. Geckos do not live on bare rock in the wild. They live on rock-strewn, sandy soil, interspersed with various types of dead and living desert vegetation. We could try to duplicate this, of course, or compromise. I personally feel that a substrate that allows geckos to dig is more psychologically satisfying and less stressful for them than being kept on a bare, flat surface. Plain sand does fit this bill, as would plain potting soil (no vermiculite or perlite), or a mix of the two. The sand is cleaner. The hazards from ingestion are probably not very high, so long as you take sensible precautions (like feeding small prey in dishes rather than loose).
Some herps are going to do an odd thing, and eat their substrate. They may be looking for trace minerals, or they may not have a reason that we can pin down. It's very uncommon, but it happens.
Whether you choose to use bare slate, paper towels, or sand, consider the other guy's point of view. You can argue until the cows come home about the merits of one of these substrates over the others, but the truth is that we do not know the full consequences of using any of them. We don't know how they affect stress levels, nor what causes a reptile to ingest so much substrate that it becomes impacted (this is surely more than the trace amount picked up with a cricket)! We don't KNOW which is "better". We don't fully understand the consequences of using each type. No one has done stress level studies, nor tried to determine whether impacted reptiles were suffering from any sort of mineral deficiency.
So rather than telling the guy with the other viewpoint that they're wrong, and you're right, try simply making sure they know the pros and cons of what they are using, so they can make an informed decision on it.
Because you could be wrong.
My other gecko Leo, got sick, i saw him poop out sand, so i removed him to a smaller tank with papertowels. He got worse cause he wouldn't drink from a different water dish, but once i put *HIS* dish in he started to drink again and looks to be much better. He's still thin but he's eating, drinking and pooping normal now. My other 20 gal tank is set up with textured ceramic tiles and an area of large rolled gravel (rolled meaning theres few if any sharp edges). I created a cave rock formation that is siliconed to a board so the gravel and large stones hold the structure in place and covers the board. I'll see how that setup works. On the other hand my baby crested gecko is on NO substrate at all. He seems to get around better on the glass than he did on papertowels and the crickets are easier to catch.
