I will tell you what the PB crew has taught me, and then you can watch for one of the moderators to fill in the blanks.

Depending on how much FA you kill, it may reduce your water's oxygen levels too much, which can lead to fish kill. SO only treated around a third at a time helps to reduce the chances of this happening. There are lots of other considerations (water quality, use of aeration, etc) which can affect how much and how frequently you treat. Hope this helps.

"Politics": derived from 'poly' meaning many, and 'tics' meaning 'blood sucking parasites'.