Dats are famous for doing this. Mine do it all the time. In the event that the frequency or force with which they are scratching increases to the point where they are actually leaving visible marks on themselves, then you can hit the tank with a broad-spectrum antiparasitic med, such as Jungle Parasite Clear. Give them a single dose and let it work for 3-4 days. Thereafter, perform a 50% wc. However, it is unlikely that you will ever rid them of this activity completely...so get used to it. In a majority of cases, it means nothing and frankly, I suspect it may be some type of grooming behavior...such as sloughing off debris or parasite-laden slime coat. If the fish do not appear to be in distress, I would just leave them alone and maintain your wc regimen.
