Lethargy in previously-active Corydoras is almost always a sign of trouble in the water that is affecting the fish. There are many things that can cause this.
Corys in my experience are not that much on eating flakes that may reach the bottom. But sinking foods like tablets and pellets are usually fine. From your info I assume they have been eating fine until recently, so the choice of food was probably OK. If you can, get a small pack of frozen bloodworms; corys love these, and it may help to get them back to feeding (along with rectifying whatever is the problem). Don't feed bloodworms regularly (daily) though, maybe 3 times a week. The basic tablets/pellets have the nutrition they need and stuff lacking from frozen bloodworms.
If the wood is clean, I would add it; corys love to browse over wood, and it may provide some "shelter" as corys all like having hiding places in the tank in case they feel threatened. A cory in a relatively bare tank will be constantly stressed, and this can lead to trouble. The more "shelter" (and they love plants too), the better.
Gravel is not a problem unless it is sharp; the laterite may be, if it is leeching into the water. Laterite is simply iron enriched clay, and iron is a heavy metal and is toxic if high enough. Don't panic, laterite is intended for under the gravel in planted tanks, so unless you added too much this is not likely the issue. But it is worth noting.
Can you give us the current nitrate test number? And, pH and temperature. These may tell us a lot, so I'll reserve further comment.
Last edited by Byron; 05-22-2010 at 06:51 PM..