fighting nitrate

So it has happened, my tanks are finally facing the final byproduct of cycling, nitrate. I changed 20 gallons worth of water in my 55 gallon minnow tank and 5 gallons worth of water in my 20 gallon darter tank with no success. How much water do I have to change to have an impact on the nitrate? Does it have to be a total water replacement?

How much nitrate is in the source water that you are using to refill? You shouldn't need a full water change to get a noticeable drop in nitrate.

It's simple math: a 25% water change should lower the concentration of nitrate (or any other uniformly dissolved substance) by 25%, so long as your replacement water has zero. Of course that's the instantaneous result. If you've got a heavy critter load or feed alot, and nothing to remove nitrate, its gonna climb back up pretty quick. I would use fast-growing disposable plants like frogbit, hornwort, najas, watersprite, etc to suck up both ammonia and nitrate. Or land plants grown hydroponically with their roots hanging in your tank, filter or sump: philodendron, syngonium, spathyphyllum, etc. But with big fish or high load you'll probably still need frequent 50% water changes.

When I was first starting out, I had trouble getting rid of nitrates. I did water change after water change with no drop in nitrates before it finally occurred to me to check the water I was using. It tested at 50 mg/l!