Fish fins problems...?

My dwarf gourami's left fin doesn't move it just stays pressed against his body but the right one works fine he moves slowly due to it and takes breaks under my ornament every now and then but his color is fine and he eats. I had a female guppy die today but other than that my my tetras, angelfish, other guppy is fine my water chemistry is nitrite 0ppm, nitrate 40ppm, any ammonia 0ppm. My momused a dissolvable weekend feeder and I forgot to tell her that it will wipe out my catfish. I had three of them but they all died including a guppy and a neon tetra