What is the problem?.., the values you indicate are reasonable for a 4 months tank, i would say even it's pretty good, why run and start to use chemicals and out materials
Reduce a bit feeding quantities, period 10-15% water changes, and you are good to go, let nauture do his thing
If you really keen on adding something in the future, check out "special blend" from microbe-lift, it smells like shit, but it's the only product with real live bacteria, dose a bit every 1-2 weeks after water change, and it will keep your tank shiny (I am not in any way affiliated with M-L, just used their shit for 3 years).