I purchased some cabomba. I placed the stem directly to the gravel with no other subtrates. I was assured that they would grow with no problems at all. They were growing for the first few weeks but now I am noticing that they are changing colour to yellowish. I have a 40 W flourescent tube in a 120 litre tank. Amazaon swords are doing great in this tank. No Co2 or other additives are being added.