I would say that 20 cm is still not very big. They would work well in a ten-gallon or a 20 long. Also, if the red is being lost in your tank, that is probably indicative of the lower light levels in your tank copmpared to sunlight. If they are growing for you, I'd still be interested in paying a reasonable price plus shipping for some. Contact me off-list if you are willing to consider. Bob Dixon