I'm a visual learner. So, I thought I'd post a picture that I took just a few minutes ago of our Retic, Thaddeus. He only eats black worms (and from time to time ghost shrimp). And his ass looks like that before eating, lol.
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Listen to these guys. They've been doing this a long time. This is no joke, and you need to take this seriously:
GET THE PLECO OUT OF THERE.
Ditch the goldfish.
Be at your LFS' doorstep
right when they open.
Grab black worms.
Feed him as much as he will eat in five minutes.
As soon as the food is gone, repeat.
Owning stingrays is a huge responsibility. Do your research (i.e. what they eat, their species, etc.) before buying your next stingray.
And around these parts, a pound of black worms runs you $20. I'm not sure about Chicago.
Oh, and what size tank are you working with there?