clams do clean the water just as laticauda said, but it would only be particle filtration, and a small amount of that. They will constantly grab small pieces of protein floating around in the water and munch on that. Although I highly doubt it would be worth your while introducing clams into your home aquarium, im sure they would put out quite a bit of nitrite which would just make you actual filter just work harder and lower the amount of fish you could safely keep in the tank. The only way I could see this being worth your while is if you had a pond and were to introduce zebra muscles instead of clams so they could reproduce quicker, but even so, im sure there are many problems with that idea. Again, this is all just my thoughts.