You want to bring it up to ~4ppm, anywhere between 3 and 5 are okay.
To do the calculations, lets convert to milliliters.
55 US gallons = 208197.648 ml
208197.648 / 1,000,000 = 0.208197648
This means 0.208 ml is 1 ppm. To dose to 4 ppm:
4 * 0.208197648 ml = 0.832790592 ml
So 0.832790592 ml of pure ammonia is how much you need to dose with. However, you're probably only going to find 10% ammonia solutions (100% is nigh impossible to get, too easy to make explosives).
0.832790592 ml * 10 = 8.33 ml of 10% ammonia solution will be what you need to dose with. This is just a little bit more than 1/2 tablespoon. My recommendation is to add 1/2 tablespoon (7.5 ml), and 1/2 teaspoon (1.25 ml).
As for the ammonia you're looking for, look for 10% solution of white ammonia. Check the ingredients, ammonia and surfactant is all it should have. If you're lucky and find one without surfactant, get it, it'll be far better. Do not get one with colorings or scents.