well i would use it for all of my tanks im just getting it because i have a coupon. for a 118 ml bottle wouldnt that treat like 200 gallons or something?
You can get syringes without needles for measuring small amounts.
Well whatever size tank you have you could measure out the 5 ml and see how many drops it is then figure it out from there. I have a 2g bucket I refill with and just use a splash per bucket, I have no idea how much I use.