Cory Missing Part Of Tail

[AnimalHouse36]

Water Parameters:
Ammonia: .15
Nitrate: 0
Nitrite: 10

Symptoms: Chunk of tail missing

I've just done a 50% water change before I noticed.

Tank: It's a 10 gallon, 2 mollys, 2 other cories. They're soon to be upgraded to a 55 gallon.
I couldn't find any sharp edges in the tank. It's a sand substrate with some gravel around the plants to keep them down.

What do I treat the tank with, and what caused this?! Would the molly nip her?!

 

[Tizer]

Nitrite 10?

You got that back to front with Nitrate?

Regular water changes and with tip top water the tail will heal.