If it is glass and has coraline algae on it, you can use a solution of half water half vinegar and a razor blade ( just make sure not to swirl go in straight directions) and it will look like brand new with in minutes. Spray on solution let sit for minute or so and spray again and scrape with pushing motion. You can use one of the 3in window scrapers in the paint section at HD or Lowes. If it is a big tank may need more than one blade. I did a 150gal cube in like 15mins. it had coraline caked on it too. It had three panes of starfire as well, and no scatches. Make sure to rinse well after, to get any vinegar out.