Hi! I am new to Fishlore, and I have a little bit problem with my aquariums nitrite. I have a 240L aquarium which my dad bought for me. This is not my first aquarium, I used to own a small tank before with some hardy fish before, so yeah, I never learn how to cycle the tank until my LFS in my area told me that I should cycle my tank, while I was getting some equipment for my aquarium and he gave me all the things to start to cycle my aquarium. So, when I arrived home I did what he told me and I cycled my tank for 2 weeks, and the nitrite that time was <0.3 mg/l/yellow color (I’m using the Tetra Nitrite Test) which was good. And after that I was getting my fishes (5 discus and 5 Sterbia corys) at the same place where I got my equipment, I asked him if it was ok to get 10 fish for one day, because I read somewhere you should get about 3 fish for the first week and he said it’s fine and I can get more fish after 3-4 weeks. Now this is where the problem started after I put all my fish in the aquarium, they were “fine” (the discus was hiding, while the cory were swimming around) the next day one of my cory died and I tested my nitrite it was 1.6 mg/l/red which was close was bad, so I change 30% of the water and put some of the bacteria, and the day the fish were fine but when tested the water it was red again and I change the water again and also the day after and after 3 days my second cory die, and I started to get frustrated and scared that maybe my 5 discus or my other cory will die. (and my mom is started to get annoyed that used too much water) Now I am using Tetra Safestart and it still show almost the same nitrite. What did I do wrong?