I would think that you have a sparse population of the two primary BB's in your discus tank given the twice daily 50% changes. In theory, an identical tank with the same fish, undergoing only weekly 50% changes, would have a more robust colony of BB; and generally a higher nitrate level. Zero nitrates could mean you have no BB and the constant water changes are preventing the build up of measurable ammonia and nitrites.
So the question is; given the environment we put our fish in, and our occasional deviation from scheduled maintenance (most out of our control); what is better:
- Very frequent water changes (multiple times / day) and sparse BB population
- Weekly WC's with robust BB colony and higher nitrates
In my case, given the work travel, many activities with the kids, and the size and involvement with my extended family, I favor the robust BB (w/ higher nitrates) that would be better at compensating for my erratic maintenance schedule.