Rope Fish Queries

First of all Welcome to AC !!! Someone here should help you, feel right at home.

Rope fish are like snakes and can eat small fishes. Count your fishes, you are probably missing one.

What do you use as your substrate?

I agree that one of your fish may have been eaten. Even when ropefish are young and smaller, they can fit some decent-sized fish into their mouths. After fish eat quite a bit of food, their bellies become distended temporarily, so that may be what you're seeing. It could also be dropsy, which could be caused by less than pristine water quality. Can you take a pic so we can better see what this lump looks like?

Ever see that video of an anaconda eating a hippo? Well, it's not that drastic, but fish do strange things that you would never think possible. Just to be sure, check your fish to make extra double triple quadruple sure.

There are many things that a lump can signify.
A tumor
Dropsy... although I dont know how dropsy would effect the physical appearance of a snake-like fish... hmmmm, it could possibly be that but I would think that dropsy would effect the WHOLE body of the fish, not just one spot
Could have swallowed gravel
Could have swallowed just about anything

If you have pictures it would help us greatly
And has Ropey been acting funny in any way (just laying at the bottom of the tank, or even staying at the top, less activity, more activity) just about anything different in its behavior is relevant.