PURPOSE: To obtain the subject additive capable of promoting fish growth by drying and grinding a mixed adsorption product of cyclodextrin and donariella alga powder to stably preserve the β-carotene in the above powder and preventing the β-carotene from oxidation.

CONSTITUTION: A mixed adsorption product is first produced by incorporating (A) 100 pts.wt. of donariella alga powder with (B) 15-50 pts.wt. of cyclodextrin followed by agitation and mixing. This product is dried and ground into fine granules, thus obtaining the objective additive. 10 pts.wt. of this additive is incorporated in 200-500 pts.wt. of a feed for pisciculture, and the resulting formulation is put to practical use.