| | Re: Need some help, plants deteriorating in my first planted tank
You can use CSM+B to dose iron, it supplies iron in EDTA chelated form which lasts a decent amount of time in the tank (though less than DTPA chelated).
The problem with using clay for supplying nutrients is clay tends to hang onto nutrients rather than give them up. The particles that make up clay are also so small and compacted that plant roots cannot properly dig through it to access the nutrients inside. Also, iron that is red (in the red clay) is in Fe3+ form, which is generally unusable by plants which need Fe2+. Soil bacteria can covert the 3+ to 2+ and so can plants, but neither bacteria or plant roots live in clay, so the nutrients stay locked away. Based on these characteristics clay offers very little benefit.
I'd warrant a guess that the silicates probably came from the soil, or perhaps the newly setup tank simple hasn't developed ways of reducing silicates effectively yet (maybe there is a species of bacteria that takes care of this?). Usually silicates get used up after a few weeks of setting up a new tank so I'd imagine the diatoms should go away soon. They also don't like bright lighting which tends to make them burrow into the substrate.
I suspect that the soil you started with didn't have a high iron content to begin with, which brand did you use?