NitrAtes

[fish4901]

I have a 75 gallon for about 7 months and overnight my nitrates went all the way to 20 and they are normally at like 5. I do a water change every 2 weeks but i don't understand. I feed my fish once a day. THe ammonia and nitrite levels are at zero. My blue tang is starting to get brown spots like by his eyes. All the other fish are healthy. This is a fish only tank. I need help. Please. Thanks!

 

[beth]

How do you your tank set up? Filters, substrate, etc.
Also, please decribe the spots you mention with more detial.

 

[fish4901]

I have a Biological canister filter on it and an off the back filter. THe substrate is seashells with some sand. It is probable 1 to 1.5 inches deep.
The spots aren't ich spots a little bigger than that and are only by the eye area. They aren't like bumps they look like indents more then anything.
I am going to do like a 12 or 15 gallon water change tomorrow to see if that helps.
I have used to copper to treat another illness one of my fish had. I want to put live rock in but that isn't going to happen now. My lfs told me that i didn't need it but I think I do.
Please help if you can.
Thanks.

 

[beth]

There is a picture of a tang in the FAQ Thread at the top of this forum that has Head and Lateral Line Erosion. Please take a look at it and see if this is what your fish has.
The type of filtration you are using as well as the shells is likely the problem with the increasing nitrates. Do the shells cover the floor of the tank? Tangs have very little tolerance for nitrates and will frequently develop HLLE where nitrates are present.