We have a 10 hp nanni diesel motor that is loosing power , what do I need to do to fix this .

can you write a better discription of what is happening ie is it during normal operation or is it getting worse each week[if so check your prop and hull for fouwling] does it get hot and loose power,is the rpm geting less slowly[bloked filters] does it use a lot of oil.

Not much info there boss. If i had to guess it would be fuel. Change the fuel filter and see if that makes a difference.