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.

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.