API has a phosphate test kit sold separately from the master kit. You may be able to find it at your LFS, but I had to order mine online. The black spots look like a nutrient deficiency, should be easily rectified by using a good broad spectrum fertilizer such as flourish or thrive, though id recommend doing research to decide which fertilizer best fits your tank. Be gentle with fertilizers, the labeled dosages are generalizations, not necessarily suited to all setups. Your substrate contains plant nutrients already, but some plants pull more from the water than the substrate and substrate does eventually deplete, faster or slower depending on plant species and growth rate.
Reinekii seems to pull from the water column, so it wouldnt get what it needs from the substrate alone. Staurogyne is a substrate puller, but it depletes nutrients very fast. We added thrive tablets under the substrate of our staurogyne when it did that exact same thing. Within about a month, the problem entirely disappeared and its now our healthiest plant species, forming a carpet even without added carbon.
Last edited by Darkblade48; 04-04-2020 at 08:17 AM.
Reason: Please use the edit function for back to back posts to keep threads cleaner