SPS need to grow a large base first, cover a decent section of rock before they grow upwards and gain thickness. Kind of like foundations of a building. Growth from existing branches and equally new branches form from the base depending on flow etc.
I reached undetectable readings on my nitrates test. I've never really obtained a decent result from a phosphate test. If you haven't any, the test reads low, if you have phosphates, something else will be consuming it and the test reads low. Doh!