It's OK. You can dose for a 1 gal tank....
1) If you have a filter, remove the carbon or cartridge from your filter, but continue aeration. (If you don't have a filter, skip this step.)
2) Fill a 1 gallon container with water that is the same temperature as his tank.
3) Add water conditioner!
4) Stir in one packet of medication. Mix/shake until the medication is dissolved.
5) Pour 13 ounces of the medicated solution into a measuring cup. Add this to his tank.
6) Tomorrow, make up a new gallon of medicated water. Add 13 ounces to his tank again.
7) After 24 hours, do a 25% to 50% water change.
DAY 3 (continued) and DAY 4:
8) Repeat the above 2 day treatment. (This will take you to a total of 4 doses.)
9) After completing the 2nd round, do another 25% - 50% water change.
10) If you use a filter, replace the carbon or filter.
This completes one "cycle" of treatment (4 doses).
If he's not better, you can repeat the entire treatment cycle. (Days 5-8. Will provide another 4 doses.)