High nitrate and dying fish

Nitrates should be less than 20 ppm for fish safety. Water changes will reduce them. Do several partial water changes ASAP until nitrate levels are down to 10-20 ppm.

PS. Partial water changes (say 25% of the tank's volume) should be done on a WEEKLY basis. One massive water change once in a while is extremely stressful to fish.

Make sure you do small partials of 25% no more than 12 hours apart. More than that can cause osmotic shock and kill your fish. Do it over the course of several days if need be.

Roan

The only way to bring nitrate levels down in a short ammount of time is water changes. To prevent this problem in the future either be sure to change at least 10-15% of the water every week or add some live plants to keep the nitrates down. (Adding plants doesn't eliminate the need for water changest hough).

How to keep Nitrates down (my personal experience)

Regular water changes (but you can only change a limited amount each time)

Live Plants (not as hard to keep as some people think)

EasyBalance (seems to work nicely, but not a replacement for water changes)

Okay, I've read their claims on the Tetra site and I'm sorry to tell you this, but that "Easy Balance" is yet another snake oil product.

There are only two VIABLE (I said viable, joephys ) ways to remove nitrate from your aquarium: plants and water changes.

NO chemical additive removes nitrate. Period.

Let me explain something: if you do not change out the water in your tank on a regular basis the KH is used up by natural processes in the tank. When the KH is used up, the pH begins to fall. If the KH gets to 0, your bacteria die.

Now and secondly, it's not only nitrates that must be removed from your tank. There are DOC, Dissolved Organic Compounds, that we cannot measure for, that are also produced by natural processes. Then the TDS, Total Dissolved Solids. The longer the water sits, the "thicker" it gets and harder it is for the fish to live in it. The only way to remove any of those is via water changes.

Now, to the Easy Balance, let me tell you what using this will do:

It will jack up your KH and pH. In other words, it will mask the fact that you are headed for a HUGE crash. It will mask the fact that your fish will eventually die in there. It will mask the fact that any new fish you add to that tank will die. Almost instantly.

What will suffer in the long run from using this is your fish. You will have your fish living in nitrifying jello.

"Hello, I'm Mr. Angelfish and I can barely breath because my water is full of pollution. But that's okay because Easy Balance is keeping the bacteria alive! Yes, they are still alive and helping me produce even more pollution!

Eventually I'll just roll over and die because I can no longer change my body to adapt to this water. Whoa. Hey, can you grab that knife and cut me out of here? It's been a few months since I had fresh water and I think the jello is starting to set."

The above is not aimed at you, it's aimed at Tetra. They make that stuff sound great! It's just snake oil.

Roan

All I can tell you is that I've checked my Nitrate before and after using EasyBalance and it worked fine for me. It did lower my Nitrates considerably. Now I'm not suggesting to follow their claim and not change your water for 6 month, but I've tried it and it works. I suppose you could use it in a situation when you have someone babysit your fish while you're on vacation and you don't want to make them go through water changes.