Water testing question/cycle question

[afish70]

Ok I have had my 90 gallon tank up and running for 6 weeks now. I started out with like 100 lbs of live sand, 110 lbs dry rock, made my own RO/Di water and mixed my own salt water. I have been testing regularly and all was looking good. On week 5 I put in 6 lbs of coraline covered live rock to help seed the other rock in the tank. Now a week has gone by and all of the nice purple has changed to brown. My test results prior to this addition were:
Ph - 8.0
Ammonia - 0 ppm
Nitrite - 0 ppm
Nitrate - 20 ppm

I did a 15 gallon water change on Saturday. When I tested my water on Sunday I received exactly the same readings as i have been getting for the last few weeks. Am I doing something wrong? Don't I need to get my Nitrates down to 0 ppm? What should i do? I was planning on getting my 1st fish either just before or just after Christmas. Where do I go from here?

[timnem70]

Many more questions need answered before being able, other than to say your tank is very new still(especially a 90gallon) to give you direction. For example;
Tanks that new are going to have some form of initial and even second and third episodes of algae blooms. It's natural. Jus keep the feeding and lighting minimum.
What are you using to export nutrients? Sump?
What's your SG, Calcium, Alk.?
Just keep going slowly.
Good luck

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[mcgyvr]

Your tank is ready to go for fish certainly.. (they can easily tolerate nitrate levels like that)
For the most part the nitrates will NOT go away on their own.. You need to do water changes,etc.. to export those nitrates.. You simply don't have enough "anaerobic bacteria" to take the nitrate and change it to "nitrogen gas",etc.. (<<-- the hardest part of the nitrogen cycle to get going)

Normal too for coralline to not grow well yet.. Its also dependent on alkalinity/calcium levels..

A tank can be cycled but that doesn't mean its "mature" enough yet as that can take months-year to really mature a tank..

You have algae cycles to come,etc... if you aren't already having them..

[afish70]

I am not feeding at all. The lighting i just started a week or so ago with "Blues" on at 10:00 am until 6:00 PM, "other colors" on from 12:00 noon until 3:00 pm and the "blue moonlights" on from 4:00 pm until 12:00 midnight. I am using a Reef Octopus HOB BH-300F skimmer, no sump or anything else. My SG is at 1.025. I have not tested for Calcium or Alk yet since i was still cycling and have no fish in tank yet. The only things in my tank are water, sand & rock, well the small pieces of Live Rock i added had a couple sponges on them but that is it.

[AK707]

nitrates dont need to be at zero just not sky high, i think anything under 15 is ok in most cases, but of course the lower the better.

doing a 15 gallon water change in a 90 gallon system isn't going to make a huge difference in lowering nitrates, if i were you i would do a bigger water change 40-50% or do a few more 15g water changes a couple days apart.

i added 2 clown fish to my tank when nitrates were at 15ppm and they are fine

[mcgyvr]

doing a 15 gallon water change in a 90 gallon system isn't going to make a huge difference in lowering nitrates

That will essentially remove 1/6th of the nitrates in the water..
So starting with 60 you would then have 50 after that water change..