I’m diagnosed having Congestive Heart Failure. I’m taking 40mg lasix. I’m told to take potassium once a day.

The minimum dose of potassium is usually 99mg but I would check with your prescribing doctor for specific recommendations about potassium supplementation.  Most people taking a loop diuretic like Lasix should take some type of potassium supplement or increase potassium rich foods in their diet.