Gfo question

[Proteus]

I am new to running gfo

My question is.

Is there a way to tell if gfo is spent.

My po4 was off the chart. To which I attribute my family feeding the tank twice a day. I was in aware of this so I fed in evening.

I have remedied the feeding schedule and done wc.
I know most People change gfo once a month or when they see a rise in po4