I have 7.5ppm nitrate tap water, my tank nitrate have been around 10 ppm. With 10% water change bi monthly. Half stocked and fed daily. 1 cu/ft sand/ pumice to 35 gallons of water. Still tinkering with the plumbing a bit, but it seams to be working.
Your test kit is not working right. If your tap water is 7.5 ppm, and doing only 10% WC bimonthly, and your tank water is only 10 ppm.
HOBs are for tanks up to like 100g appropriately stocked and fed conservatively.
You can hang 4 double size HOBs on 6 ft 125 to 200 g tanks for a combined flow of at least 1400 gph or 7X to 10X turn over rate. Additionally, you can add circulation pumps inside the tank to increase the flow. I use 3 Penquin 350 in my heavily loaded 125g and that's all I need.
And take rocks...not all are porous. Many aquarium safe rocks are not very porous at all so a bio-mass is left to populate the surface only. Plastic decor will also provide surface area.....but in the end, large tanks with lots of fish will require some sort of man made bio-media
Life rock isn't porous as bio growth will seal the surface just like live sand is only aerobic on the surface 1/2 inch. The undulating surface of rock and substrate provide many times more bb growth than bare bottom tank. All you need to provide is good flow over the life sand and rock. Bare bottom tank may need external bio media but it doesn't necessarily have to be man made. Many reef systems place life rock in the sump.