I just acquired a 100 gal acrylic tank, and it has old dried on coraline alge all over it. Was looking for any ideas on how to remove it with out scratching the tank.
I would spray it down with some distilled vinegar and let it sit for a little while then use an old credit card to scrape it. You can take a towel and soak it in vinegar and lay it over any really tough spots, sounds weird but it works well.