In some cases, you can break/cut it apart and estimate the amount you need. Or you can dissolve it. For example: 1 tablet treats 10 gal, so:
1 gal tank: It would be difficult to break it into 10ths. But you could dissolve it in 10 ounces of tank water, then pour 1/10 (which is 1 ounce) back into the tank.
2 gal tank: Dissolve in 10 ounces of water, then pour 2/10 (which is 2 ounces) back into the tank.
As long as you use the correct ratios, you'll be OK. Ie:
1 gal tank: Dissolve it in 32 ounces of conditioned water. Then add 1/10 (3.2 ounces) back into the tank.