I much prefer using ammonia, as once 30 seconds of math has been done, you just add it and and wait.
30 seconds of math:
= 40litres(ish) = 40,000ml
1/1,000,000 of 40,000ml is 0.04ml (i.e. 1ppm of 40,000 is .04)
4ppm then is .16ml
ammonia strength is what, 8% (that's what mine is), so 0.16/8% = 2ml
so 2ml (just under 1/2 tsp) of 8% ammonia will bring a 10 gal
tank to just over 4ppm ammonia.
so much less gross than pulling a rotting shrimp out. But you have to make sure the ammonia is pure (well 8% ammonia gas dissolved in 92% pure water) and doesn't have any surfactants. You do that by shaking the bottle; if bubbles persist it has them. I use Old Country ammonia