It is Chilodus gracilis, sometimes called the Black line headstander, which grows to about 3-1/2 inches.
A 10g tank is not suitable for Molly's as they grow too large and have heavy bio-loads. Headstanders also are not suitable for such a small tank along with the fact they are very sensitive to water parameters so doing large weekly WC's to keep toxins and nutrient levels low are mandatory.
Usually the fish in the shops are sold as the Spotted headstander (Chilodus punctatus) but this fish is the similar looking Chilodus gracilis, sometimes called the Black line headstander. The biggest difference between the two is the prominence of a dark band running along the middle of the fish, which in C. gracilis is always present, but comes and goes on C. punctatus and is most obvious during breeding behaviour