That large of a water change might shock the fish and kill it. If it's been kept in lousy conditions for a long time, it's become acclimated to those conditions. Changing it over to a clean tank all at once actually does not work in your favor. Sad, but true.
Hope it all ends up well.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~