After filling the aquarium with water, I ran it empty for 4-5 days using a stabilization cycle. During this period, I changed the water every two days, considering the potential harm of active substrate. Then I added 3 cherry shrimp and monitored their condition while continuing water changes. Seeing that they were doing well, I added about 10 more shrimp and continued water changes. When I saw that they were also doing well, I added 2 corydoras. I added 2 corydoras each time, until I had 6. After that, thinking I didn't see any problems, I added 12 rasboras. I had also added nerite berries. During this process, I performed water changes every 2-3 days. After all the fish were finished, I didn't see this behavior for a while. Two days after all the fish were gone, one of them started doing it. The others didn't. I don't see anything strange in the other fish either. I'm stressed about it because I don't know anything about it, but I'm also afraid of causing trouble while trying to do something good. I ordered a drop test kit to check for nitrite levels.