Substrate affecting water

I have inert regular gravel with very little flourite mixed in. I read a post that had indicated thet the substrate was like a battery and phosohates would return to where it was before water changes were performed.

I am wondering if my gravel being an older tank is having this same affect. Up until recently I was using an undergravel filter, I have since discontinued. I believe my no3 is 40PPM and phos at 5PPM.

I have been thinking of switching the gravel out for soil master select. Is there anything to the substrate acting as a battery continually recharging the phos and or nitrates?