Fishkeeping - Nitrate Shock

Phil wrote:
I've read a few times in the past posts about being careful of 'nitrate
shock' from doing large water changes on tanks that have high nitrate
levels.
Could someone fill me in on a safe level of nitrate reduction over a given
time and also links to any articles that explain the 'nitrate shock' theory?

I believe the term you're actually looking for is "old tank syndrome".
When a tank has not had water changes in a long while and the organic load
has built up (as indicated by high nitrate levels), the buffering capacity
of the water has often been consumed, causing a pH crash. This means the
pH is fairly low. Now, there's two main concerns with doing a large water
change as a consequence of the fact that you are likely adding higher pH
water with some measurable amount of buffering capacity. First, there is
the pH shock itself from the rapid increase in pH. Secondly, at low pH,
ammonia is converted to a less harmless form. When you raise the pH, the
ammonia converts back to the lethal form, potentially causing a lethal
ammonia spike.

So the recommended method is to thoroughly test the water parameters (both
tap and tank) first and make several small frequent water changes instead
of one massive water change; something like 10% daily instead of one
massive 50-75% change. Aim for no more than 0.2 to 0.5 (the closer to 0.2
the better) pH change every change. Monitor the ammonia levels closely and
have detoxifiers on hand (Prime, Amquel, etc) in case they start rising.

Now, if on the other hand the tank has high nitrate levels, but has had
frequent water changes and there has been no pH crash, then you can
proceed at a fairly normal rate of 25-30% per water change, but I'd do
that every other day or at least twice a week rather than just weekly
until the levels come down. Now, if the tap water has nitrates, you'll
never be able to get it lower than the tap value through water changes
alone. You'll need to use supplimental things like live plants and nitrate
absorbing resins.