I'm sorry, I completely disagree that you can't do more than a 25% water change. I do 50% weekly and so do plenty of other people. If your tank is toxic, change as much water as you can to keep the fish safe. There's absolutely nothing wrong with a large water change at all. Prevailing wisdom is that the more fresh water the better. If the temp is the same, it won't hurt your fish. The amount of BB that lives in the water itself is almost negligible. It's your filter that matters.
This may sound odd, but did you shake the nitrate test bottle #2 really well? And the test tube after you cap it? The API kit is a little strange in this, but failing to really (REALLY) shake that bottle will give you off readings. Also make sure you're comparing the color in bright, white light against a white background. I add an index card behind the test tube next to the test column; the little gap between the columns isn't wide enough, IMO. The nitrate colors are very close to each other and can be hard to judge.
Finally, don't forget about your ammonia. It should be 0, not .25. IMO, that's more important than your nitrates at this point because it indicates that either your BB aren't doing their job, or your tank's not clean enough (leftover food, fish waste, etc.).
My advice would be to NOT change any of the media in your filter except your carbon. Carbon usually loses its effectiveness in about 4 weeks. (In the long-term, look into getting re-fillable bags, it's much cheaper. Or you can take your carbon out, clean it, and bake it. That's the only way to re-use it.) The carbon doesn't actually affect your water chemistry in the way you need. Lots of people don't use it at all. I use it for greater water clarity and because my water smells without it (it's very hard, alkaline water).
The only important BB you need to worry about is in your filter and taking any away right now would be a very bad idea. While there is some BB in your gravel or on your plants, the only stuff that matters in any significant way is in your filter. I'm not sure what kind of media you have, but you probably have a sponge of some sort and maybe bio-beads or ceramic rings. These should be cleaned, not changed. And generally you clean only one at a time (if you have more than one). I think someone has already mentioned to clean it in tank water.
Here's what I would do:
- Condition (with Prime) and test your tap.
- Test your tank. Be sure to shake Nitrate #2 AND your test tube really, really, really well.
- Do a 25%-30% wc. Make sure the temp is the same. I recommend an instant-read digital thermometer. Meat thermometers at the grocery store are cheap and accurate. You can set the temp right out of the tap.
- Wait 24 hours.
- Test your water again.
There really should be a change in both ammonia and nitrate readings at that point. If not, it's possible that your tank is going through a mini-cycle for some reason. If that's the case, the best choice is small water changes every day until the numbers are right.
All that said, these are my opinions. I think I am fairly well-educated on this subject, but feel free to throw it out the window if you like.
