Yesterday I lost a honey gourami with no obvious signs of disease and today i lost a cardinal tetra which was stuggling to swim normally but wasnt showing any other obvious signs. i have no idea what is happening. I have tested the water, the results are 0 ammonia, 0 nitrites and 5ppm nitrates and all the other fish are currently looking well. Im not sure what to do so any help would be appreciated.
A water change mightn't go amiss - might dilute any possible toxin undetectable by a home testkit, and certainly wouldn't do any harm (unless your water supply company mentions some sort of works going on in the area).