Water Change???

I just recently transfered my already estabished 90 gallon set up to a 75 gallon.(because of the 90g was leaking due to worn silicone) anyways I used 35 gallons of water from the 90g tank and 40 gallons of store bought pre mixed water. It has been about 4 1/2 weeks since I set up this 75g. I just checked my nitrates and they are high (80 ppm) Now I wanted to do a 60 gallon water change to bring my nitrates way down! One pet store said thats too much to do at once and another pet store said yeah do it!! Ok so I just did a 25 gallon water change it brought my nitrates down very little to like 65-70ppm I want to do a big water change like 60g IS THAT TOO MUCH OF A WATER CHANGE ALL AT ONCE OR NOT??? /DesktopModules/ActiveForums/themes/_default/emoticons/blink.gif

 

My nitrates where high in the 90g from using tap water probably over feeding lack of knowledge (I'm still learning about this hobby that I'm addicted to) I was doing small water changes on the 90g but I was using tap water. Ok I did not use the live sand I had in the 90g I used all new sand (not live) I was told that my live rock had enough life to spread to the sand. When the 90g started leaking I took the live rock/coral out and put them in a couple of smaller aquariums I had I used the same water all the coral lived I actually even noticed growth in some. I do have a skimmer it's a bubble magus NAC 7. And I purchased a RO/DI system so at least now my water is pure. I just wonder how much of a water change can I do at one time...