Yeah - Pirarucu is the safest bet as any other fish in that location is still high enough to give you at least shards, crystals, dyes and lot of clear demi even if it will not increase your desynth level anymore or you can flog those on MB for some extra profit if you don't want to bother with desynthing thwm. I've actually got The Dissasembler tile purelly on CUL with majority of the fish caught at Proud Creek ...