Your nitrates are high. Will be an issue if it is approaching 30+ ppm. What is your Kh? That combined with the Ph will give you an estimate of CO2. Could also be some bacteria or parasite the new fish brought in.
A water change will lower the nitrate and help remove any other dissolved solids. Melafix is a helpful prophylactic when adding new fish though you'll need a real med if you suspect infection.
David
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