Holes in leaves of geranium Roseanne

Help please - I'm a newly enthusiastic gardener and still learning - my geranium Rozanne which I brought about a month ago is doing really well with lots of flowers and has spread already but it's leaves are covered in tiny pin prick holes. I have a slugproblem but they decimate the plants so donthink it it's them as the holesso tiny tiny! Any ideas anyone? Thanks