High nitrates should go away with enough water changes.
That really depends on the source of your nitrates. you may still have an abundance of die-off on your LR
that may not have been fully cured. With time, however, if this is he case your levels will drop as a larger bacterial colony establishes to cope with the excess nutrients from the LR
Another thing is that you may be overfeeding your livestock which will constantly keep your nitrate levels high, not to mention phosphates too. Water changes will only help the situation if nothing is being done to cause these high levels to begin with.