Peacock eel bleeding?

[lalalameg]

I have had this eel for a little over a week, it was a gift from some friends that I didn't particularly want...He's 5-6 inches in a 20 gal tank (which I know is too small) with gravel substrate (which I know should also be sand). I thought I'd try and give it a go with him anyways. I've been feeding him frozen blood worms that seem to be being eaten (in the morning their gone) but last night/this morning he was acting kind of weird, lethargic and not too shy (he usually hides when I come near the glass and now I can get right up close to it and he doesn't even move). The biggest issue though is probably these red streaks on his tail. It looks like internal bleeding or something? His only other tank mates are 3 platys. I will probably return him to the pet store but I thought I would get some thoughts first. Thanks! (I've attached a few photos below)

 

[ShawnKC]

He could be damaging himself when burrowing into the gravel. I made a similar mistake with a pair of Fire eels, I had similar sized gravel as a substrate and they would burrow and scratch their flesh up. Eventually they developed skin infections and died before I could change the substrate to sand.

 

[lalalameg]

That's what I was guessing...just the other night he also tipped a rock over and I'm wondering if that could have potentially fallen on him :/