Hi, I have 6 Harlequin Rasbora's and 1 has a swollen underneath and red spots around the sides (see photo) I also have Cardinal tetras, rummy nosed tetras and amano shrimp. They all seem fine. Water params all look pretty normal too. Ph7, ammonia very close to zero and nitrates about 15-20. Any advice what it is likey to be? Thanks!