I'm the past when I had fish I would change out the 1/4 of the water every other week to keep the growth hormone low so the fish would grow faster. Once the water is saturated with GH they stop growing. How does the chemistry in an AP system work with this? Does I still need to change out some of the water or do the grow beds dissipate the GH?
Build up of growth hormone in water is a debated topic. Many of us have seen good growth without water changes. Here's a link that may help. http://www.plantedtank.net/forums/showthread.php?t=226874