It will take more than a couple drops.
= 200L (subtracted a bit for rocks)
200 L = 200,000 mL
= 0.2 mL
= 0.8 mL of pure ammonia
Of course ammonia is a gas at room temp/pressure so it is dissolved into a solution at around 10-15%
0.8 / 15% = 5mL or 1 tsp
So add about 1 tsp of a 15% ammonia solution and see how long it takes to be converted into nitrites. Don't worry if nitrites gets up to 10 ppm
it won't hurt the bacteria.
You are done when the 4ppm ammonia is converted into nitrate within 24 hours