Nitrates high-Is this deadly?

[newt5005]

I have a 90 gallon tank. It has a false back, with a protien skimmer, bio-balls and the main pump that is contained in the rear chamber. The tank had been running for approx. seven years. I purchased it in a fish only condition (yes, it had sand) about 3 years ago. I have been adding live rock slowly. I have also added some misc fish, crabs and turbo snails. Over the course of the last year, almost everything has died, but not at the same time.
Here's some more background. I struggle to keep the temp at or below 80. The salt level is almost always at .022. There has never been any measurable Nitrites. Never been any measurable ammonia. I have 50/50 lamps that are on about 6 hours a day. Currently, the Nitrate level is over 160 ppm. I run 20 gallon water changes around 3 weeks.
The aquarium has right now, one cleaner shrimp,around 1 1/2 years old. One clown fish and one blue damsel, both around a year old. About 8 or 9 crabs, purchased on 2 seperate occaisions. I feed frozen shrimp 1/3 cube a day.
I did 2 water changes 3 days apart last week to try and get the nirtates down and it helped a little. Is this the reason all the other fish (and snails) only live a few weeks?
Imput from some of the veterans out there would be helpful.I have tried to provide as much background as possible, but if more info is needed, I would be happy to oblige. I want to add some fish that would get to around 3 or 4 inches, maybe larger, but am afraid to at this point. Thanks. Mike

 

[mbm28]

Nitrates are unlikely to be the killer. First, test the testers. Double check your thermometers, your pH meter, your salinity meter (hydrometers are terrible), you chemical test kits, etc. If everything is still as you say it is, it very well may be the nitrates but I still would not think that. There is an advanced topic in the hobby called vodka dosing (it is as it sounds) which can effectively lower your nitrates and phosphates to reasonable levels relatively quickly. Your other options are a denitrator (commercial or DIY) or start growing macro algae in a deep sand bed refugium. Hope that helps!

 

[cool99bomb]

nitrates must be EXTREAMLY HIGH to do any sort of damage like he said check everything, including alkalinity and calcium. If your alkalinity and calcium is low then your fish will die all the time. Everytime you change the water you lower the alkalinity and calcium so remember to add more

 

[newt5005]

I have purchased new chemicals / kits to test with twice over the last 2 years, and have taken in samples to the local fish store, all with the same results. I keep 2 thermometers. One stick-on to moniter the bottom of the tank, one floater for the water on top.
I have not however purchased a new device to test the salt content. I assumed ( I know, I know) this was something that was a constant and did not 'go bad'.
Am going to read up on the Vodka thing, although it sounds a little risky. Will have to Google denitrator as nothing comes up on this site in the search mode.
Could you elaborate on the macro algae? I thought live rock had this in it. All the live rock in the tank is (suppose to be) from Fiji.
Something I left out with the origional post, the inside of the tank starts forming green algae after about 7 days after the water change. Is this about right? Also, not that it might matter, but I have brown disc looking things growing on the rocks. I had thought this was a good thing. Now am questioning if it is or not.