Check out VeipaCray's thread on a way to add water without the need for Prime:
viewtopic.php?f=9&t=33171Many of us use that so we don't need to expose our turtles to the chemicals in water treatments. That being said SeaChem says you are safe to double the dosage, but why do you want to?
If you are just doing a water change and not using a hose system, treat the water before adding it, and you'll not need to turn off the filter. Even doing a 20% water change and treating after all the water is in, should not seriously hurt a mature filter.
The chemical reaction between Prime and chloramines is almost instantaneous, so if you do turn off the filter, you are safe turning it back on after dosing.
For the first few months after getting my turtle, I lugged 5 gallon buckets and added prime to them. I never turned off the filter and did not have problems. Once I got a python system, but before I added the inline carbon filter, I didn't turn off the filter and added about half the prime halfway through filling and the rest at the end. I didn't have any problems with a cycle restarting. I've used the inline carbon filter for about 3 months now on both the turtle tank and the fish only 20 gallon that the original turtle tank turned into. I've not had to use prime since.
I'm hanging onto it, in case I do end up with a mini-cycle, but water changes are so much easier.
Posted: Thu Feb 28, 2013 10:35 pm Seachem Prime: Dosage and waiting time
I'm using a hose system for water changes,and do em 50% at a time monthly! So i'll just off the filter, dose half the prime needed while the tank is filling up and add the other half when the tank is filled up...