Plastic Zebra Table Covers What Causes Sudden Death Of Newly Added Zebra Danios To A Fairly New Tank?

What causes sudden death of newly added Zebra Danios to a fairly new tank? - plastic zebra table covers

During the last few months, I have an aquarium of 30 liters of fresh water. I added a school of Neon Tetras (7 + fish) and a school of Tetra Glolite (about 7 fish). Yesterday, after I read something, I decided to get some zebra danios, after hearing how they are hard. So I set about 7-8 of them and they) acclimatize to the tank (after floating in the bag. Today morning I was on the lookout for two deaths and one (in the process of dying as with seizures and other's). N nitrite, ammonium nitrate or the present and the pH is over 7.1 to 7.2. No idea why they fall so-called big fish community dead?