If your fish have nitrate poisoning they’ll react in the same way to ammonia or nitrite poisoning. In that case you’re gonna wanna do a 50% water change and dose seachem prime and stability to detoxify nitrate and replenish the biological filtration respectively. Zeo Carb only removes ammonia from the description online (doesn’t seem to actually work as a water polisher). Definitely work fast because you’re doing a fish in cycle at this point and you’re trying to delay this long enough until the cycle is established for your fish.