I have a 75 gal fish only tank it has been up since fathers day it started with the contents of a 54 gal corner tank more live rock was added along with more sand it had a mini cycle and has never showed sighns of ammonia or nitrites since. I started doing water changes it was set up with tap water and has had some allege growth and red stuff growing on the sand so I was also doing water changes on my 29 gal reef tank so I decided to mix up and change the same amount in the 75 gal I changed about 30 gals in the last week and a half. the water has a brown tint could this be the ditmiats or the allege dieing off from the nitrates coming down from 150 to 2.5 to quickly? I think this is what is happing I changed out another 10 gals tonight hoping to help the ammonia back to 0 it is curtinly between 0.50 and 0.25 about the same as the min spike it had during the min cycle. I have bought a ro di unit but it has not came yet. so I will run by wallmart tomorrow and get some more. i need to do another weekly change in the 29. Poor fish have to go without supper tonight