Sinkers as they are known - are Goldfish and Koi which rest on the bottom of the pond. Let's divide these into two classes which are treated differently:

Class A - Sinkers which have to struggle to swim into the midwater or to the surface. These fish may have a swim bladder problem, sometimes due to bacterial infection, sometimes due to defects in air bladder filling and some have had an autopsy following death to find water in the airsac instead of air.

It is rare thank goodness, but, Faulty electrical equipment can cause 'electrocution' which can cause this symptom as well.

Class B - Sinkers are fish which swim fine when disturbed. These fish usually feature a massive fluke infestation and are being weakened by these flukes. They rest on the bottom, which further assists the flukes in becoming numerous on the skin and gills - the fish eventually dies. 

Treat this fish by giving it a salt bath initially at a dosage rate of 7% to 10% (3/4 to 1 ounce per gallon) you should, as always when giving any fish a salt bath, ensure that you have at least one air stone in the bowl with the fish being treated.

Hopefully the salt will kill off many of these flukes and you can return the koi into the main pond .. but it would not harm to also salt the pond at the rate of 5% (1/2 ounce to the gallon).