Connecting Piping in Your System – You can use a silicon based glue, but keep in mind you will need to get the Aquarium Safe version as well have patience to let it cure. The better way to do this is to drill your hole slightly smaller than your fitting, heat the rim of that hole up a wee bit with a welding torch and screw the threaded male fitting through the warmed hole~ no silicone is needed when you get a nice tight fit. Practice on the top of the bed for an overflow hole if need be.
Helping the pH Balance Adjust – Calcium Carbonate or Potassium BiCarbonate are the two primary buffering products to use and both readily available at the local Home Depot or Lowes. Calcium Carbonate is available for rural supply shops horse suppliers etc. Shell grit will do just as well although more a slow release buffer over time. Some people use “slaked lime” and or “hydrated lime” Calcium Hydroxide which breaks into Calcium Carbonate in the presence of Carbon Dioxide. Gypsum can be used to adjust the “hardness” (a measure of Ca/Mg) of your water lemons, vinegar and Hydrochloric acid can be used to move pH in the other direction…. use with care and caution… and very discretely…. 1ml in 1000l will move your pH by 0.1….
Pump Size for System – The purpose of moving the volume of water through the grow beds is to allow the bacteria to work. There are other reasons such as solids removal and oxygenation, but that is secondary. If you have a 1000 litre fish tank, operating on the 45minOFF/15minON then you will need a 4000lph pump as a minimum assuming minimal head.