"old school" razor blade attached to a semi-rigid tubing works fine for me. Sadly you cannot make the tubing to long as it becomes too flimsy to apply correct pressure. To attach the blade to it: slice the tube perpendicular to its opening, slide the blade in, then a drop of super glue. Easy to replace too. Perfect for areas where corals are making it impossible to put a magnet. And will remove the toughest coralline.
The hardest part is finding said blades nowadays...
The hardest part is finding said blades nowadays...