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A specific pathogen free (SPF) filtration system was built to exclude Haplosporidium costale, Seaside Organism (SSO) from the Pacific oyster (Crassostrea gigas) hatchery and nursery of the Molluscan Broodstock Program at Hatfield Marine Science Center in Newport, Oregon. Since its inception in 1999, all groups of Pacific oyster seed produced have tested negative for SSO or any other certifiable infectious agent under histological examination. Microparticle filtration to 1 μm, further filtered to 0.35 μm for larvae, or in combination with ultraviolet sterilization, for juveniles was used in the system to exclude SSO from seawater drawn from Yaquina Bay, where it is a resident. The SPF water had negative effects on larval and juvenile oyster growth, and juvenile survival. A series of experiments assaying different water treatments was conducted in order to remedy these effects. The addition of calcium bentonite and …