Nitrate level depends on whether your tank is planted or not. For planted not good... plants use nitrates for their growth. Their source of nitrogen. Somemore can cause BGA growth (ugly). But if it's not planted.. 0 nitrate is fine. With Yamato.. as long as conditions are stable and within healthy levels they're fine. If there's sudden changes in water conditions.. yamato start acting funny... like climbing to the water surface and hanging out there... trying to escape.

To increase nitrate level.. Add KNO3... potassium nitrate. It's available in powder form.

Increasing bioload and ensuring good biological filtration will also create nitrates when Ammonia is broken down into nitrite and then nitrate.