Hi,
Yep gonna need that other information and of course a pic would be great, although it does sound to me that he does have a tumour, these fish are unfortunetly prone to them, they seem like white balls that just grow bigger, but lets not panic until you give us all the information and the pics,
When you said you clean his tank every 7-10 days, you don't clean it completely do you? because you should only be changing 25% of the water and making sure you clean the filter only when neccessary and in the water you have taken out of his tank for the water change, that way you keep all the good bacteria in the tank, and you keep your fish happy and healthy