No need to put new water in. Prazipros medication chemical formula breaks down (why there’s a specific amount and timespan to dose). Then after treatment throw a bag of carbon in and if you wanted to be extra extra sure before you stuck the blenny back in you could leave the carbon in for a couple hours. Remember that if you don’t leave a skimmer running to run an airstone to add extra oxygen
Also, in the future if you ever need to treat your blenny, you’ll need chloroquine phosphate (which you should quarantine tank for because the algae blooms are aweful- the phosphate)You can ask your vet (or in my case I contacted my local aquarium and tracked down their vet) who was kind enough to prescribe some.