The assumption that widely reported reconstituted water [1] associated problems in cladoceran bioassays are due to unrecorded, unidentified, and unisolated materials is unwarranted. It is possible that distillation/deionization systems could add materials to the waters they produce, but the production of substances of sufficient toxicity in sufficient quantity to exert measurable interference in the culture and testing of Cladocera is rare. The generally reported difficulties with reconstituted water (RCW) [1] more likely reflect a problem inevitably associated with RCW or with any simplified medium. By fiat it contains only four salts. The full complement of trace elements demonstrated essential to Eukaryotes is missing. Assuming that this omission will be corrected by unknown additions of trace elements from contaminants either in food materials or in the few salts intentionally added is initially appealing but imprudent.

To satisfy the animal's needs and to assure test validity, trace elements must be both present in sufficient (but not excessive) quantity and consistently available. Evidence, based on work with zinc and selenium, suggests that, at least for these elements, the latter requirement is not readily met by solid foods because the preferred route of uptake is via the liquid in which the animals dwell. With this in mind indications that the U.S. EPA is considering requiring RCW both for maintenance and testing of invertebrates are disturbing. It is likely that other essential trace nutrients will behave in a fashion similar to that of Zn and Se. We have found that employing A-MS algal medium with carefully timed harvest of food algae and planned feeding regimes can minimize the damage introduced by general use of RCW. While there is considerable room for improvement, the nutritionally more complete A-MS offers clearcut advantages over U.S. EPA and O.E.C.D. (Organization for Economic Cooperation and Development) media.