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While the partial water changes remove some of the nitrate, it is quite hard to get nitrate to below 5ppm without doing multiple water changes every day. Perhaps you are still not doing the nitrate test right.
You need to keep ammonia at 0ppm. It can be the cause for flashing since ammonia can burn fish's gills to make them feel uncomfortable. Feed them only once every two days if you have to.
There are other parasites beside ick. Some can't be seen by naked eyes. However, you shouldn't rush into any conclusion at this point, since most of the parasites killing meds are also stressful if not harmful to the fish.
Just keep the living conditions at optimized level for your fish, while keep eyes on any changes.
Best luck to you ☺☺
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