What he said.. . Instructions are on bottle...I used this stuff, its hydrogen peroxide based but it lasts in the res for up to 5 weeks whereas the straight peroxide I think you need to add to the res every couple of days as it's not stable.
Silver Bullet 1L - Gold Coast Hydroponics
bactericide virucide sporicide fungicide algaecide amoebaecidewww.goldcoasthydroponics.com.au
Feel the roots.... are they slimy and soft or not slimy and firm?
If they are firm you'll be ok... with dwc you need to either have a completely life free water by using h2o3 or you need beneficial microbes to protect the roots. Beneficial can give you ph issues so I fo completely clean