Thanks. I'll get some.0.4 mL per day.
Or if you have a good dropper, 8 drops is about 0.4 mL
An old test syringe, rinsed out, will work perfect also. This amount gives you .25 for 20 gallons, so if 15, it goes to about .35 for ammonia level, still not too high for any critters in the rock.