The source of nitrates and phosphates is essentially the same for both. However, remember that these are dynamic systems-- so while the levels initially start out the same, nitrates and phosphates get used differently and at different rates, and it is strongly dependent on the algae type and bacteria and so forth growing in the tank-- and even what stage of growth they're in. SO, while initially you could see a correlation between nitrates and phosphates, sooner or later one will be left in excess (usually nitrates) and the other will be consumed because the stuff growing in our tanks needs more of it (phosphates) than it does the other thing (nitrates). PLUS, there's a whole lot more nitrogen in waste than there is phosphate, so that's why phosphate is usually the limiting parameter for algal growth.