Why? These are called "geophagus" or "eartheaters" for a reason. They almost constantly take large mouthfuls of the substrate and sift it through their gills. A sand substrate is perfect for them. Pea gravel will tear up their mouths if they try to do this. I would highly recommend changing back.
Also, as was mentioned earlier, these are more likely G. abalios, not surinamensis. True surinames are very seldom seen, especially in stores. abalios and altifrons are pretty regularly mislabeled as surinamensis