scratches on tank

[glen.1984]

ive noticed some hairline scratches on my tank recently but not sure weather it was from the guy who had it before or if it is my common plecs which are scratching it up? when i clean the glass (which isnt too often) i use a cloth on the inside of the tank and make sure i dont disturb the sand to cause the scraches so i dont think thats the problem. so its either that they were there all along and every now and then i notice them or the plecs are doing it.......... so i guess my question is can common plecs scratch glass