2 drops per gallon

if your bottle doesn't have a dropper lid, you can buy a pipette or an eye/ear dropper.
Droppers can be bought at any drug store for a few dollars, usually in a pack of 2 or 3.
I bought a 100 count bag of disposable, 3ml pipettes from Amazon for about $5. These are awesome because if one gets accidentally contaminated (by soap or whatever) I can just throw it away and grab a new, clean one.
I do wish the manufacturers of these products would list amounts with smaller tanks in mind. Seems every bottle I pick up says something like "for every 100 gallons..."