Easiest one I can think of to get you started is Virus by Iron Maiden. Just follows the vocal melody, very simple.
After that I think some early Metallica ones are pretty easy, never learned any of those myself.
I think this is the answer. The tank not being cycled is obviously a problem, but not one that will kill off relatively hardy tetra overnight. Chloramines will. You need dechlorinator.
This is where you reply "Give it 56 minutes or so and it should be there".
Repeat as needed.
I'm not familiar with the Go (last Line 6 product I used was the original GuitarPort) but do those amp models include cab sims? If they do, you're running a cab model into a real cab and that will ruin the sound. Unless there's a way to turn the cabinet simulation off, you need to run into a power amp (which your effects return is) into a full range PA speaker, not a guitar cabinet.