Well, technically yes.... You can take a packet and divide it into tenths. Then use 1.5 tenths to treat your 1.5 gal tank.... BUT I would worry about getting all ten "tenths" to equivalent amounts. If they aren't equal, you run the risk of over/underdosing.
(I've done this type of process, where I divided meds into tenths, but I have access to a balance/scale. I was able to weigh a single packet, and then measure out 1/10th of the packet.)
The reason it's suggested that you dissolve the med in water, then measure out the water is because this gives a better approximation of the correct dosage. For example, if you dissolve one packet in ten ounces of water, you could then just add 1.5 ounces of water to your tank.
Re the post that said to use 5 cups of water then more water to equal your tank size for one packet. This will only work if your tank is 10 gallons. Otherwise, as pittipuppylove said, you'll end up with an overdosage of the medication.
Here's what I would do:
Find a measuring cup that that shows you either 10 or 20 ounces.
a) 10 ounces: Add one packet. Fill to 10 ounce line with water. Dissolve the meds. Remove 1.5 ounces, and add to your tank. (You'll have 8.5 ounces remaining in the measuring cup.)
b) 20 ounces: Add one packet. Fill to 20 ounce line with water. Dissolve the meds. Remove 3 ounces, and add to your tank. (You'll have 17 ounces remaining in the measuring cup.)
Last edited by LittleBlueFishlets; 05-02-2013 at 08:45 PM.