Well folks, here's we are!
Ammonia: 0
Nitrite: 0
Nitrate: 20.
In the end, I ended up getting a second Aquaclear 50 for my tank, thus allowing me to use my Tetra Whisper 10 as seed material.
After a good PWC, Bildeschoen seems good! Also, my sister bought some BettaFix and his tail fin has...