2nd large 50 percent water change and follow it with another 50 percent water change 2 days later if in fact the nitrate's are up. I'm going through the same problem... Getting better tho
I've also had great success with carbon dosing for the last 6 months. My nitrates and phosphates are close to 0 in a moderately stocked 20g long with 15g sump. This is the 2nd time I have gone back to carbon dosing - the 1st time it didn't work out so well. From my experience, the key to a successful long-term carbon dosing is using an automatic doser instead of manual dosing. You can get cheap dosers now which do the job quite well.
I had a neglected tank that I revived. Nitrates were high did massive water changes but couldn't get them below 40. Started dosing with vodka. On a maintenance dose now and nitrates zero. Worked great for me