You don't have to drain. I started with regular gravel, then after having limited success with the plants, added some flourite. I just dug up plants, removed some gravel, replanted and replaced gravel around plant with flourite. I did this while doing water change, so mess was minimal. The water got cloudy from the flourite but it cleared up quickly.
Then during every water change I would remove some gravel and replace with flourite near the plants...I probably have 50/50 flourite/gravel now.
I noticed a difference. Plants are definitely rooted/growing better.