You do need to get rid of the staghorn that's there. If you're inhibited in overdosing Easycarbo because of the shrimp then you'll have to attack it during water changes. Use a spray bottle or paint brush with the liquid diluted. JamesC suggests a 3:1 water to liquid mix. Spray directly on the expose areas when the water level is low, wait 5 minutes or so and then refill. Do this several times a week.
Your filter outlet also seems as if it's just an open tube pointed along the long axis of the tank. I'll bet the plants on the far right are doing OK. Your problem however is on the lower left. The Koralia is good but the CO2 concentration will still be much weaker. This appears to be a distribution problem so I would try the spraybar mounted along the back wall so that your effluent comes towards the front glass and will be deflected down towards the substrate. Might also want to continue playing with the diffuser position as well.