Since water is so basic a substance, it is hardly necessary to point out the seriousness of this ruling. Even in an urban setting, it is easy to envision realistic scenarios involving jeopardy to the health of especially vulnerable observant Jews, not to speak of lesser but nonetheless deeply troubling consequences.
There are rabbinic decisors of stature who disagree with this stringent stance, relying on an attested and respected opinion recorded by earlier authorities that the water is permissible if the organisms in question are not discernible. (This is apart from the ruling by at least one distinguished rabbi that the copepods are permissible because of the halachic status of reservoirs.)
I make no pretenses to any standing in a debate among poskim on such a question, but I believe it is important to underscore certain considerations with the hope they will help encourage the latter authorities – while maintaining the highest reverence for the great rabbis who hold a stringent view – to keep the halachic discourse on this matter alive.
There is strong reason to believe that the presence of these crustaceans in the city water supply is not a new phenomenon – and that the almost universal, instinctive reaction that water imbibed over the years by a host of tzaddikim and talmidei chachamim must be permissible does not deserve to be dismissed as irrelevant.
Under analysis, this instinct has two separate though related dimensions. The first, less rigorous one, is a reflection of the celebrated Talmudic assertion that God does not cause a stumbling block through the agency of the righteous. Though this is arguably an aggadic comment that cannot in itself drive a halachic decision, the Talmud applies it in legal contexts, and Tosafot (Hullin 5b s.v. tzaddikim), after subjecting it to questioning, concludes that it applies specifically to the consumption of forbidden food, which would be especially unbecoming for a tzaddik.
The second point emerging from the fact that this water was consumed by generations of distinguished rabbis carries greater – perhaps even compelling – weight. There is a well-known principle that the Torah was not given to the ministering angels (lo nittenah Torah le-mal’akhei ha-sharet). The recent psak alludes to permissive rulings issued in the past regarding truly microscopic organisms on the grounds that it is impossible to avoid them (I efshar lehizzaher). Strictly speaking, this is only partially true. One can filter very many microscopic entities, so that if the criterion were simply what can or cannot be avoided, the argument could be made that those permissive rulings should be limited to organisms that cannot be removed by any feasible method of filtration. Why, then, do we react to such a proposal with the unequivocal rejection that it deserves?
The real point, I think, is that from the time of mattan Torah until the invention of the microscope it was impossible to know that these organisms existed, and the notion that God would have forbidden something that no one could know about for thousands of years, thus causing wholesale, unavoidable violation of the Torah, offends our deepest instincts about the character of the both the Law and its Author.