The quote below is from Inland Fisheries Informational Leaflet No. 35 as posted to the State of California Department of Fish and Game's website.


"Tilapia will be approved only only in San Bernardino, Los Angeles, Orange, Riverside,San Diego, and Imperial counties. It may be stocked there only in waters approved by the Department of Fish and Game. Only T. mossambica and T. hornorum may be stocked, and recipient waters must be approved by the Department. No tilapias may be imported until the genetic integrity of the stock has been certified. This certification may be required for every shipment."


Although I don't know for certain I believe that koi ponds are exempted from these requirements under this paragraph in the publication above:

"Aquaculture" means that form of agriculture devoted to the propagation, cultivation, maintenance, and harvesting of aquatic plants and animals in marine, brackish, and fresh water. Aquaculture" does not include species of ornamental marine or freshwater plants and animals not utilized for human consumption or bait purposes that are maintained in closed systems for personal, pet industry, or hobby purposes, however, these species continue to be regulated under Chapter 2 (commencing with Section 2116) of Division 3."






JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)