How do you use flroakleen ?

I run a 55 gal res, I drain and fill it every week. I want to use florakleen say every 4 weeks. Can I just add to my res say day 6 with my nutes and let it run for 24 then drain and start over? Or do I have to drain on day 7 then add new water with florakleen let it run for 24 drain then start over ?
Hope that makes sense
Thanks

I just drain the res and clean it. As it fills with new water I add floraclean. Then when full I add nutes. change it out next week. I use it every change and haven't had a problem