I have an Acrylic Tank and the safest way that I have found, is that good old American Express Gold. Use a credit card, it will not scratch the acrylic, but it's strong and sturdy enough to take out even small scratches.
Funny how the good old American Express Gold will work for a lot of things in this hobby.
Jokes aside, the credit card will work better than any scraper that I have found.
Good luck...!