Just to report back on the first set of water tests today.
I attached the Pozzani nitrate removal filter cartridge to the tap, and flushed it for at least 2 minutes, as suggested in the instructions. I left it running at a low flow rate while I found my test tubes, lids, and rack.
I took sample of filtered water, then removed the attachment from the tap, and took a sample of tap water.
I tested both samples with the JBL test strips, and with the API Master Test kit. I have attached pictures, which may not be great, but at least you can see the difference between tap and filtered water.
The test strips show lower pH and KH for the filtered water, no difference in GH, and much lower nitrates.
The API tests show lower pH and nitrate for the filtered water.
I could afford a drop in KH, but don't want it to drop too much for fear of it impacting tank water stability. I could always increase KH slightly with baking soda, or use a mix of tap & filtered water.
I also have a filter cartridge to reduce hardness, so my next step is to attach that to the water supply (in line with the nitrate filter), and test the water that comes off that. I'm hoping it will lower GH without lowering KH. Fingers crossed.