At night is better generally, although you can train fish to eat at pretty much anytime if you're stubborn enough. Also remember, those Algae wafers are HARD! It takes them quite a while to soften up in the water. I have a tiny little 2.5 inch common pleco. He can suck on it, when I first put it in, but that little guy isn't going to gobble it down, it's as big as his head, and he just doesn't have the teeth for it. Put it in when you turn the lights out, overnight it will soften, your pleco will come out as normal, and in the morning, it will likely be gone. You'll never be sure if he's eating it or not, I know my Colombians, well one of them anyway, like chewing on the Algae wafers as they swim around, so they help break it up too. Those Wardley's ones are the ones I use as well.
Most fish don't mind putting on a show for you when they eat, but remember, others like the privacy to munch on something when its dark *shrug*.