Why are my goldfish resting on the tank floor?

I have 6 goldfish and a white cloud minnow in my fish tank. Ive noticed that for the last few months they will go and rest at the bottom of the fish tank untill I disturb them by going near to the tank or turning the light on/off. Before anyone suggests it, the tank is big enough for them. I have had a look over them and there is no clear sign of disease (finrot, fungal infection, white spot etc) The only problem I have had is abit of fin nipping but when I noticed this I put some aquarium salt in the tank and then a few days later some anti fin rot - fungus medicine in to prevent the fin from becoming poorly. They are feeding normally (practically snatching the food from my hand lol) and the water is clear and clean as I have a good filter. The PH level is normal and there is an air stone going so there is plenty of oxygen. Are they just resting or is there a problem I need to be aware of? When there not resting they seem feisty enough?