It looks great. Plants look healthy (despite slow growth). Add more fish and go crazy. As long as the plants are green and growing, and the algae isn't, keep doing what you are doing.
I would also do water changes alittle more frequently than 1/3 of the substrate bottom every other month...what does that amount to in gallons? The evaporation could affect your GH, driving it upward if the loses are significant. But I agree, your plants are in a steady state and need food to grow...start with increasing the KNO3. On the other hand... your tank looks good!