I can't seem to get my PH to stabilize above 6.0 anymore. In my previous experiences with Aquaponics my PH usually hovered around 6.2 but this time around it appears to be around 5.0, sometimes as low as 4.6. I just recently installed a heavy particulate filter which is essentially a rubbermaid tote filled with 3/4 gravel from my driveway (it was cleaned of course). The water is pumped into the rock bed and the flow-outs are higher up, so my theory is that the heavy particles will get stuck in the rock and also sink to the bottom while the cleaned water will flow back into the flood table. Anyhow, the water has cleaned up a lot but there is still quite a bit of heavy fish waste and leftover food sitting in the flood table. Is this the reason my PH stays so low?

Is there any other reason besides the accumulation of solid waste that could be causing the low PH?