First, yes it's safe. If it weren't then the 10's of thousands or more of us who use a higher FC level in our pools would all be ill and we would not be maintaining our OWN swimming pools in such a manner.
The CYA combines with the Chlorine and produces bound up chlorine. This bound up chlorine is NOT active. Some of the chlorine added to the pool and in recommendations with TFP chart is not bound by the CYA. This unbound FC is the ACTIVE chlorine and is quite low. As the active chlorine gets used up, the Bound Chlorine releases from the CYA and become active.
The difference between TFP recommendations and Pool Industry recommendations is this.
The Pool Industry recommendation for FC is to kill bacteria - only, and then shock once a week to kill algae and to use algaecide.
TFP recommended levels will kill bacteria AND control algae but that be in relation to whatever the CYA is.
TFP recommendations are MUCH less expensive way to maintain your pool and it is safe.
The Pool Industry and Pool Chemical companies has known about this FC/CYA relationship for decades, but refuse to recognize that it's effective. If they did, they could not sell all of the chemicals that they do, and would go broke.
Here a couple of links to discussions on the subject. If you are curious about chemistry, then this should interest you and will explain the chemical processes.
Questions about Chlorine and CYA
Pool Water Chemistry
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