noting it down here:
The smallest neon tetra was gone, the whole medicine regiments did not help. It was lying flat belly on the ground. When I used long chopsticks to try to get it out, it swam away side way. In order not to get other five tetra to the same sickness, I had to scope it up and put it into another bucket where all the left over plants were (with filtering system on). But it seemed disappearing into the bucket.
Now, aquarium status: 1 Betta 5 Neon tetras.