I have a 58 Litre aquarium (apparently 19G?) with 3 Peppered Corydoras which I plan on increasing to 6 and a BN which I'm planning on rehoming as soon as I can. I'm hoping to find an algae eater that is happy on its own and will go in my tank.

I thought so, but someone told me it was actually 19G. Dimensions are 24" long by 12" wide by 15"deep

Okay, dimensions tell me you have a 19 gallon tank (72 liters), so I'll stick with 19 gallons not 15 gallons. Dimensions don't lie. So, up the peppered cories to 8. And yeah, rehome the BN.