Hi all,
It doesn't really make much difference to Plecs (other than the ones that bury themselves in sand banks), but sand is the best substrate for Cory's. The best sand is the rounded silica swimming pool filter sand, but play sand is OK (unless it is dust fine).
One thing to remember is that Gee (and Jozebs) are fish breeding gods, and have a lot of experience (I haven't), meaning that they see things that other people don't (I don't), allowing them to get away with bare bottom tanks etc. If you haven't got that level of fish-keeping ability than a sand substrate (even a thin layer) has the advantage of helping with water quality and tank stability.
So sand has 2 major advantages, it is the natural substrate for most Cory's and they feed by sifting through it, and it provides a "home" for many of the microbial processes that stabilise water quality. Have a look at the section on "structure and substrate here: <
http://plecoplanet.com/?page_id=829#BOD1>
You need to follow a slightly different approach to gravel if you use sand. The intake for the internal filter needs to be well up enough in the water column (or with a rock placed immediately under it) so that it doesn't suck the sand in (personally I like a coarse sponge over the intake), and you don't need to/ can't really clean the sand with a syphon.
If you have a tank with a fair degree of flow you can still use sand, just mix in a small amount of fine gravel, the flow will then sort the substrate and leave the gravel in the areas with high flow and deposit the sand elsewhere where the flow is lower.
To get around the problem of waste accumulating you need to move the filter(s) around until you get a slack spot somewhere accessible (You can also use a large rock to break the flow), then put a piece of slate at the slack spot (you may need to cover it with rounded pebbles). The slate or slate and pebbles acts as your "dustbin", if the current is too strong for the waste to stay on the slate, add the pebbles. You syphon your waste from the slate, the rounded pebbles should still allow you to syphon out the waste out from between them.
I keep planted tanks and MTS, (if you have MTS on their own they may do) and I've never had any problem with sand and gas build up. This is from the Skeptical Aquarist
http://www.skepticalaquarist.com/docs/nutrient/sulfint.shtml (always well worth a read)
"
In sum, hydrogen sulfide could only be an issue in a substrate that was too deep (over 4 inches, say), that was entirely anoxic, was also depleted in nitrate and was enriched with decaying organics and sulfate, perhaps from fertilizer. Then, to get the H2S up into the water column, though, you'd have to get in there at long intervals and vigorously stir up the deepest layers of substrate with a gravel cleaner. My point is that several poor aquaristic practices would have to be combined. "
Again in my experience adding plants to the aquarium is probably the single most important thing you can do in maintaining water quality.
cheers Darrel