Be sure you have a mortar mix, not a stone aggregate mix. Follow manufacturer directions as to how much water. Don't make soup or any other suggestion. Concrete does not stick well to styrofoam. Making it wetter and weaker will not help in the long run. Wind nylon yarn around the foam and using a knitting needle, fasten it where you can't wind it around. Lay the background on its back and apply the concrete where it will stick. Don't worry about 100% coverage. A second layer should connect the dots. I would use the acrylic fortifier because it makes the concrete smell better while you are working with it. That is enough reason to use it for me, but it should also make it hold on in thinner layers than plain concrete. It probably also seals in some of the chemicals that will gradually make the concrete strong when protected from being leached out. If they do leach out, they can raise th pH too high for fish comfort.