If 80% still shows nitrite off the top of the scale, you need to do another very large water change asap.
Try diluting some tank water with tap water then testing that for nitrite. It won't be very accurate but at least it will give you a rough idea.
Start with 50:50 and if that's still off the chart, try 4 parts tap to 1 part tank and multiply the result by 5; if necessary do 9 parts tap and 1 part tank and multiply the result by 10.
It is possible to use salt to prevent nitrite harming the fish in addition to water changes. The calculation is in this thread. https://www.fishforums.net/threads/rescuing-a-fish-in-cycle-gone-wild-part-il.433778/
Try diluting some tank water with tap water then testing that for nitrite. It won't be very accurate but at least it will give you a rough idea.
Start with 50:50 and if that's still off the chart, try 4 parts tap to 1 part tank and multiply the result by 5; if necessary do 9 parts tap and 1 part tank and multiply the result by 10.
It is possible to use salt to prevent nitrite harming the fish in addition to water changes. The calculation is in this thread. https://www.fishforums.net/threads/rescuing-a-fish-in-cycle-gone-wild-part-il.433778/