The total number of days in 3 weeks is 3 * 7 = <<3*7=21>>21 days.
If 2 koi fish are added every day, the total after 21 days is 21 * 2 = <<21*2=42>>42 koi fish.
if 5 goldfish are added every day, the total after 21 days is 5 * 21 = <<5*21=105>>105 goldfish.
The total number of fish in the tank after 3 weeks is 280 + 42 + 105 = <<280+42+105=427>>427 fish.
If there were a total of 200 goldfish in the tank after 3 weeks, then the number of koi in the tank is 427 - 200 = 227 koi fish.
#### 227