Go and get a product called safe start.... Since you already have the fish it is your best bet at not killing another one... If this one dies. Dont get another.... For a while. Im going to try tp break this down for you as best i can. When fish poop the poop releases ammonia. It is toxic to fiah and will kill them. Within your filter padding is beneficial bacteria or bb for short. In a immature tank there is not very much bb but as the tank cycles the bb have time to grow. When the amonia in the water pass through the bb it gets turned into nitrite, then it turns to nitrate. If there is not enough bb then the amonia will not be converted thus killing the fish.... This is qhat has happend with your other too and off your not carefull this kne will join them..... Do your research before buying ANY other fish... Or ANYTHING for that matter
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