If you're having a difficult time keeping your plecos alive then you may want to reevaluate some of the basic aspects of its care - like how much food it is receiving. While most plecos are sold as algae-eaters, some of them are very specific when it comes to the kind of algae they like to eat. Your fish tank can grow quite a few different species of algae, and since you pleco probably doesn't eat all of them it may be slowly starving to death. This also applies to snails and other algae-eating critters as well.
In order to prevent them from starving you should either A) have enough of the proper kind of algae for them to graze on, or B) supplement their diet with some sort of green vegetable. I personally like using shelled peas and zucchini. However, you can use just about any time of veggie to feed them - from broccoli to leafy greens. With a little bit of experimentation, you'll quickly find the food that your pleco or snail prefers - most of them love spinach leaves. With the exception of the leafy greens, I would recommend tossing the veggies in the microwave with some water for a few seconds so that they are easier to tear chunks out of.
While algae wafers are a viable alternative to fresh veggies, I really don't like using them. They tend to puff up and disintegrate long before the fish have finished eating, which leaves a nasty mess behind for you to clean up. However, if you like using them that's perfectly fine! Just remember to break an appropriately sized chunk off of the wafer to feed your fish - the whole thing is way too much.