After the tank is fully cycled you can add livestock. The term "cleanup crew" is really something created for marketing purposes. You really don't need snails and crabs, or sand stars...They don't really clean anything. I don't have them in my systems. I might have a crab or two because they are cool to look at, but not like these websites say you need. It's just ridiculous. Whatever you add will need food to sustain them. If you have an abundance of algae, you have high nutrient levels. If you have food on the sand bed, you are overfeeding and don't have enough flow.
If you want snails and crabs, add one or two and see how that goes.