I wouldn't use any kind of scrubber on an acrylic tank, only a soft sponge.
I have used a dish sponge/scrubber in my tanks, and it works pretty well. Just make sure it has never been used and rinse it well. Credit cards are good for getting green spot algae off, but I wouldn't use that on acrylic either.
If you get some money together eventually, the Mag-Floats are great, but cost about $10.