Coralline algae on Acrylic

[Pokerman11]

I try to keep the acrylic clean on my 90g DT, but there are a few areas that I've neglected for a year or so where there is a pretty good build up of coralline. Mostly on the sides at the bottom corner. At first I thought it looked great, but now it's something I want to get cleaned.

I've tried credit card, fingernail, acrylic safe brush but this stuff is like purple concrete, it 'aint going anywhere.

Any suggestions?

 

[heathlindner25]

A piece of acrylic, with a fresh 90 degree cut on it works great