Midwest2: I am waiting for a talmid of Rav Belsky to respond to your question where we can find the original psak. The CRCWEB dot org cite has a number of piskei halacha that were heard from Rav Belsky directly including:
Copepods in New York City Water
Submitted by: Rabbi Yaakov D. Lach
Rav Belsky permits drinking New York City water
without a filter. His primary reason is that the reservoir,
by definition and in function, is a bor (water pit) and
therefore the creatures that develop there are
permitted. Although the heter of a water pit is only to
drink straight from the pit (since when removed from
the pit, the creatures are considered “piraish” /
separated and become prohibited), in our case this is
not a concern. This is because in New York City all the
creatures show up dead at the tap, due to
chlorination. There are Rishonim who are lenient and
rule that once dead, the creature remains mutar,
even when separated. We are allowed to follow this
opinion when there is a tziruf (additional lenient
factor). Here, the tziruf is that the incidence of the
creatures is not very common, generally being only a
miut hamatzui. Rav Belsky himself drinks unfiltered
water.