Phosphorus is often seen as second only to nitrogen, in its importance to plant growth and development. Dark red or purple leaves are the tell-tale sign of phosphorus deficiency. Stunted growth with reduced leaf size and quantity is also observed in phosphorus deficient crops.
Discolored purple leaves Stunted growth
Phosphorus levels will often correlate to the number of fish in your system, though natural phosphates can be added if levels are found to be insufficient.