The problem with pH readings of 7.6 or 6 is that they are at each end of the scale and as such it impossible to tell if they are actually beyond those levels. More on water testing aquaponics systems later.
Yesterday I discovered a white powdery substance near where I collected my gravel. I had a better look at it and discovered the gravel was dissolving! It was near where my flashing battery tail light for my mountain bike had rusted out so the conclusion I choose to leap to is battery acid!
I spent the afternoon washing out the system a few times, and now the pH is sitting at 6.8.
I was just a few days away from adding my fish so I guess its a good lesson to learn at this stage.