IMO!
That's the tricky part.
Test your tap water first.
If you're going to use compressed CO2, then (according to Mr. Barr) you want to make SURE that your kH doesn't fall below 3. Same with gH. You may not even have to worry about it, mine comes out of the tap kh=6,gh=13.
Po4 (phosphates) are fertilizers that you will most likely have to add. Some folks tap water already contains decent amounts of phosphates, again; check your tap water first.
kH and Po4 will be depleted faster than gH. kH you can replace with measured doses of baking SODA. We tend to use Fleet Enema for dosing Po4.
Read the Fertilization Sticky at the top of this forum for more specific dosing levels. Po4 should range between 0.2-0.5 for high-light tanks.