Summary
It's not just nitrogen that makes plants grow. Phosphorus is also needed for plant growth and development, with stunted growth being the most common symptom of phosphorus deficiency in crops.
Phosphorus Deficiency can be seen as second only to nitrogen when it comes to importance in regards to plant life - a tell-tale sign would be leaves turning dark red or purple due from lack of phosphorus while stunting their own rate of growing; this often results in reduced leaf size and quantity too.
Symptoms
Discolored purple leaves Stunted growth
Preventive Measures
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.