Limestone and sea shells are both composed of Calcium Carbonate, which will react with Phosphorus in your nutrient solution and precipitate out as insoluble Calcium Phosphate, which is not freely available to your plants. Thus, using limestone or sea shells as a medium is not a good idea. If you think you have some limestone in your medium, here is how to test for it. Pick out a few suspicious white lumps (limestone is white, unless contaminated) and put them into a plastic cup and add a few mL of Dilute Sulphuric Acid; if it fizzes, it is probably limestone. The best course of action if the medium is contaminated with limestone, is to discard it and select a better quality medium. Alternatively, you could try washing your medium in Dilute Sulphuric Acid, and wash well after with water, and watch where you drain the washings. This is the end of Simple Sand Hydroponics . The Author hopes you enjoyed it enough to actually build your own system. If you need clarification on any points, pl