I am half way through a water treatment but my nitrate has gone up what do i do?

nitrates at 250 are at a lethal level, so i myself would do a water change. its not worth treating the fish if they all going to drop dead from poor water quality anyway. id do a 20% treatment today. leave it a few days then re-test. once the retest came back safe, then i would carry on with the medication. but i guess thats just me. im not sure what the "proper" method of approach would be here