My nitrites are off the charts 10.0+ i have done water changes & they don't seem to be coming down?

Ok I have a new tank...about 4 weeks with a fancy goldfish ammonia is 0 nitrites were 10.0, now 8.0, nitrAtes were 80 but are now normal, pH is 8.4, alkalinity is 300mg/L, hardness is 150mg/L, chlorine and chloramine is 0. I took it for an initial testing and did a 40% water change (the pet store said this was not a good idea) then did a 10% water change at night then another 40% (new person at petstore told me that is was the stores policy to advise us not to change water) after talking for a considerable time with the "expert" he agreed that i should change some more water because nothing had changed in the tank (but he never said that ;) ) sadly we lost a fish... so i changed out another 40% the next day and the nitrites dropped to 80mg/L (got the new guy again at the store) again he said to not change the water (wink wink) i changed another 40% last night then another 40% this morning...i dont want to lose my FG. the test strip show + that the good bacteria is developing...what should i do??? should i leave the tank alone now or continue doing a 10% water change until it is normal???