If I'm doing a 25% change on a 30 gallon do I add enough conditioner for the 7.5 gallons or 30 before I add the water?
this can depend on both, how you dose the tank, and on the conditioner used.
if you add the conditioner to the tank, prior to adding new water (conditioner dependent) you dose for the total tank size. if you dose the water you are changing (conditioner dependent) you add for the size of container you are using. (or for the total of water to be changed).
conditioner dependent, it is many hours before the conditioner is "used up". this means it does not really matter how you dose, as long as you follow the instructions given, by the product. as time goes by you will get a "feel" for the best way for you. you can also add the conditioner before or after adding the new water.
few tips though:
at first, follow the maker instructions. only experiment as you gain experience. even if you are told, here or anywhere else, of a different way.
having said that, lol. we, often recommend here to dose, up to, to twice the "proscribed" amount.. again only experiment when you know your conditioner.
use a conditioner that, also, has heavy metal binding additives (its the only reason i use a conditioner).
dont be "conned" into buying an expensive conditioner. Prime, for example, works out @ pennies per tank, even very big ones. (this seems to come out best if you ask posters here, who have any length of experience that is). other, well known, conditioners can add up to £'s, per change, if you have a large tank..