Water snails are widely recommended as being essential to pool hygiene, and the suggested stocking rate is 1 for every 2 sq. ft. of water surface area. I must admit that over the years I have become doubtful whether their value is real enough to justify introducing them deliberately.
They do consume a certain amount of decaying vegetation, but are not above eating the soft growth of some aquatic plants. Frog-bit foliage suffers badly and the big pointed snail Limnaea stagnalis will go through some oxygenators like a scythe. The tender littleof Nymphaea tetragona and N. pyg-maea alba have no chance if there are snails present. Snails also to some extent on the short furry growth of some types of algae but make little real impression on it. They may be scavengers of fish droppings and other detritus but they produce waste themselves. They certainly do not clear green water as some people believe.
On balance they do perhaps more good than harm but they – particularly Limnaea stagnalis – need watching and thinning out if they overproduce. The Black Ramshorn Snail Planorbis corneas disappears from most of the pools into which it is introduced, partly I believe because it is more specialised in its environmental requirements than limnaea, and frequently because of the depredations of leeches.
If there is some doubt about the desirability of introducing snails into the pool there is none in my mind where mussels are concerned. They are definitely not a good thing, at least in artificial garden pools. For one thing they like a layer of mud in which to embed themselves, and that is not present when containers are used. For another, their larvae attach themselves to fish and live as parasites,on the fish’s body for about three months. This may not be fatally harmful to the fish but it is obviously not the sort of thing you would want to happen to your favourite goldfish and shubunkins. Mussels filter algae out of the water and would in theory be able to clear green water, if there were enough of them. But if the water is completely cleared of algae, what can the mussels do for lack of sustenance except die? A Swan Mussel can grow to a length of 8 or 9 in. and contains a lot of meat. When a creature of that size decays it pollutes the water as few other things can. No, mussels are out.