so you removed 10% of 50ppm which should be 5ppm....
Not sure about the accuracy of your test kit, but 5ppm on my nitrate kit would be almost impossible to tell a difference in the color once it gets above 30ppm.
I'm sure you know this already, but i would try rinsing any mechanical filtration off to remove trapped food/waste in the water column and do atleast a 10% WC.
Every other Wednesday I change 25 gallons in my 55 gallon using a continuous WC method. I pump water in at the same time I'm taking it out. Its a little less effective, but over the 30 minutes or so it takes. It makes any shift in salinity, temp, etc less noticible to the creatures in my tank.
Sounds like alot of work, but it is actually quite easy. I just mix the water up in a 30 gallon garbage can let it run for about 30 minute (I know you are suppose to wait a day), then start the process.
I also know most don't, but i choose to vacuum the substrait for the first part of the WC.
Brandon