Water change water change water change! The number one way to reduce nitrate levels. A 9g change on a 55g is only removing 16% of the nitrates; not much at all. A change of 30% (16-17g) or so will make a little more of a dent, but obviously won't make a huge diff if only one is done. I wouldn't do a change over 50%
Nitrates do not leave the tank unless they are removed (or if its a well planted tank; the plants will use the nitrates as food). It will not correct itself. Think of it this way: anything you add to the tank STAYS in the tank unless you remove it, although it may be in a diff form. Nitrates are the last form of the nitrogen cycle.
I would suggest at this point, because your nitrates are well above dangerous levels (nitrates are not generally dangerous like ammonia or nitrites, but its suggested to keep levels below 40ppm), daily water changes of 20-30%. After a week you should have manageable levels of nitrates. Do be sure to gravel vac the first big change (definitely do a 30% or more change), and every other change thereafter for that week to pull up any rotting food/poo/detritus in it. I'd start tonite. Good luck!
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.