Hi;
I assume you've got the full range of micromesh, up to 12,000 grit, and that you're wet sanding with all them, right? What I've found, and I've only used the stuff on black sheet and clear tubing, never clear sheet, is that once I get to about 8000 I'm basically ready to go to a buffing wheel. If you don't have access to a buffing wheel, or at least an arbor on a drill, I guess you could use a palm sander, wet sanding up to 12,000. I've gotten black sheet to look pretty good hand sanding with micromesh, but it always looks better after the buffing wheel. If you're not wet sanding that's the problem, but my guess is that you already know that. Whatever you do, you want to make sure you don't get the acrylic hot.
Oh yeah, I've only polished cast acrylic. If you're working on extruded, I don't know how it would polish, but it's a lot softer than cast.
Good luck. Polishing a whole tank by hand with micromesh could take a LONG time.
HTH
Matt