I don't really understand this reply because ammonia and nitrites should be zero in both tank and tap..? Keeping nitrates at zero is not needed even for discus.That makes sense, thank you for that information. But due to the sensitivity of discus, I have to keep ammonia nitrates and nitrites at 0ppm.
If you want to get better water quality I think the best way is to take same fish out, because looking at your stocklist it is not really moderately stocked, but heavily stocked. Those 2 Synspilums create a lot of waste and the bigger they get the harder it is to keep your water at great quality for your discus. It will probably just get frustrating, especially since the Synspilums may start to stress your discus since they are a little too much to keep as tankmates.
But if you want to try, I'd go for 2x or 3x 50% water change a week.