Be sure the size of the fin is for the size of motor. Don’t put one on that is only for substantially larger motors than you have.
It’s best if you have electric trim, because with a fin installed you should really trim often and be aware of your trim setting.
Too much speed with too much down trim will cause the boat to lean to one side or the other while going straight. Too much down trim will also cause the boat to lean a lot more in a turn at cruise, even though it takes a larger area to turn around. This can be very scary. I have put the rubrail of my 19 ft I/O in the water doing this.
If it leans too much…….trim up.
If it porpoises…………trim down.
Hope this helps.