Cardinal tetras getting bent tail?

Hi, I have 5 cardinal tetras in my tank and they've been fine for about 2 months now but recently i noticed that 2 of them are getting bent tails. Its strange cause the tails are bending downwards and it looks really weird and the fish looks like its been bent. The two that have it are getting worse but the others done have it(yet). What should i do? i have a 40 gallon tank and my ammonia, nitrite and nitrate are fine. I have some tetras, gouramis, corys and a blue ram. Is there any medicine that can help? thanks