The link doesn't give a list of ingredients - does the bottle have this? You need something that contains ammonia or ammonium hydroxide, water and nothing else. Sometimes they add detergent or perfume, which make it unsuitable. If it doesn't say, put some in a small see through container with a lid - do this outdoors as you do not want to breathe it in - and shake. If it makes big bubbles which burst quickly it's good to use. But if it makes foam, it's got detergent in.
The link does say that the concentration is 9.5% which makes it easy to work out how much to use. There is a calculator in the How To Tips at the top of the page and the last item on there is for calculating how much ammonia to add. It's preset with most values, you just need to enter the volume of your tank.
