It looks like a syrphid fly larva, or "rat-tailed maggot". They are not parasites. They graze on biofilms and decaying organic matter, usually in low-oxygen aquatic environments. The "tail" is a breathing siphon to allow them access to air. Your stock tank full of slightly decaying wood made an ideal egg deposition spot for the adult flies.
Clown loaches need more room than you've got. Botia loaches (yo-yos, zebras, kubotai, etc.) are similar but smaller.