Are fresh water clams harmful? Someone told me they help to clean the water.

--
Don

True, they DO help to clean the water, but one would need to stock
them in a ratio of one for every 2-3 gallons of water for them to have
any effect on water clarity... also keeping in mind that they are
living, breathing organisms just like the fish and/or snails...
remember that they also consume oxygen and contribute to the bio-load
in the pond. On top of that, they release insane amounts of
contaminants when they die, more so than fish; and it is often
difficult to tell when they die as they don't keel over or float like
a fish... you'd have to pick each one up and visually inspect (and
smell) it to determine whether or not it is still living. Therefore,
it would me my recommendation to not keep them, or if you're
absolutely bent on doing so, keep just a few as a curiousity. If your
pond needs filtration, you would be best off to purchase a good-sized
foam filter with a pump sufficient enough to turn over the total
volume of your pond twice in an hour. Some disagree with this
formula, but I've found it much more effective than lesser GPH to
volume ratios.