Hi there, and congratulations on the new tank!
I always treat with dechlorinator for the entire tank, not just the water I am replacing. I measure the temperature of the water at the sink to make sure it matches the tank. I use Prime brand dechlorinator, because you use less, so the bottle lasts a long time. It treats 50 full gallons with just a capfull.
think i have the same kind.
You don't need to actually measure the 10 or 25 percent exactly. That is just a rough guideline of how much water to change. Just look at the tank, and 25 percent will be one fourth of the way down. It's okay if you take out a little more or less. It is not dangerous to add too much dechlorinator, so again, estimating is fine. Always err on the side of too much rather than too little dechlor.
this is what im probably gonna do.
It is ESSENTIAL that you get a test kit during the cycle. Ammonia in fish urine will burn the gills of your fish and kill them, which is why you need to test your water and do regular partial water changes until your tank has cycled. The cycle will grow colonies of bacteria that will turn the ammonia in the urine into nitrite (still dangerous) and then into nitrAte (less dangerous). Until the cycle completes, the levels of ammonia and nitrite can rise quickly and poison your fish.
You should buy a liquid reagent test kit (a lot of people here recommend Aquarium Pharmaceuticals Master Kit). At the very least you will need tests for ammonia, nitrite, and nitrate. You can follow the process of the cycle and do water changes whenever ammonia and nitrite levels rise above about 0.5. You don't want to let them get past 1.0, or your fish are really in danger. First you will see a spike in ammonia, then in nitrite. When ammonia and nitrite fall to zero and you have only nitrAtes, the cycle is complete, and you will have to do water changes less often.
i currently have a kit. but its the dip stick kind. is that ok or the reagents are better?
I agree with everyone here who said don't add any chemicals except dechlorinator. The fish store will try to sell you a bunch of junk that will only cause you more problems.
I also agree that a Python is the best investment you will ever make for your tank. It makes maintenance so easy. You will never regret it.
am yet to hear anybody who has a bone to pick with the PYTHON. guess i am really getting me a 50-60 foot one tomorrow, another food variety. think its like "worm" thing.. and a filter media.. guess the hydor ceramic as suggested.anything else i should get guys? 
Good luck, and keep asking questions.
