I am having a horrible start to the day - fish die off

Hey gang - I have to post because I'm baffled and upset. Well, sort of. I know WHY this hapened but I took steps to fix it immediately.

So I woke up this morning and found my beautiful Threadfin Acara was dead, then I saw my Jurupari a few inches away also had passed, then my Firemouth in the back of the tank... all perished.

I immediately moved two of my other *important* fish into my 36 gallon as I had to run out the door for work and didn't have time to do a water change.

Anyhow, my tank is going through some sort of mini-cycle. Ammonia is at .25 and nitrites were off the charts, well above 1.0, so I know that water quality was the problem and caused this.

However, I've been doing daily 20-25% water changes, and last night I had done a 50% water change and I have been dosing the tank with Prime in the meantime to convert the Ammonia and Nitrite to non-toxic form while the water issues resolve themselves.

To this point, it had been ok. I'm not really entirely sure why the tank has a bacterial crash/recycle process occurring. I had added a few fish last week, and perhaps the bio-load was too much? I added about 6 or 7 Swordtails to provide live food (when they breed) for the Cichlids... there was a day that I lost power and my filter was off too, so perhaps I lost some bacteria there.

Anyhow, I'm upset because it wasn't negligence. I've been testing water everyday, and as soon as I saw the issue I responded immediately as best I could with frequent water changes, and Prime to condition the water. I don't understand why the fish died given the fact that I used Prime... I thought that was supposed to lock up Ammonia and Nitrite? I totally understand that Ammonia and Nitrite are deadly to the fish, so I understand that the presence of these in my tank are potentially fatal, I just don't understand... why my measures didn't save the fish. I did what I could as soon as I saw danger and I thought I had taken the correct course of action. I'm so bummed out right now.

Advice?

Also - Oddly... my Julii Cories and Ags Cories which were both wild caught, seemed completely fine. Not breathing heavily or anything. Perhaps this is because they can dive to the top and gulp air if necessary? They weren't doing this at all though. Additionally, temporarily, my 36 is now way overstocked while I resolve this issue. I'm afriad to put anything back in the other tank until it finishes cycling again.

Thanks Mark - I did not clean the filter before bringing it back online because I thought it best not to disturb the bacteria in the filter...

I'm absolutely heartbroken about this. I had these guys in my tank for over a year from juvenile size, and they were thriving. Showing incredible color.

With my Nitrite now off the charts, how long would it possibly take for this number to drop? Should I leave the water alone and let the bacteria eat away at it as is, or continue doing daily water changes, say 20%?

Thanks for your advice. Oddly, I actually pulled my bubble wand out of that tank last week when I did a water change because it had become so gunked up and I didn't think if was necessary because if anything I have a filter that is rated for a larger tank and provides plus water circulation.

It is a good idea though to have a battery operated aerator on hand though so I'll have to pick one up

This is just bugging me because I saw my Jurupari near the surface a few times yesterday and I knew that was a red flag and immediately changed the water as I had been doing the few days before as well. I just can't wrap my head around what happened over night since I had also used a dose of prime. I wonder if the Pimafix I used for the Swordtails had anything to do with? Doubtful, but I had used a dose of that as they had broken out with a bacterial infection due to lousy water parameters. A salt bath cleared them up, but I used Pima as well to soothe them in the tank. The Prime use is what I don't get. I've used it in the past to neutralize toxins when odd circumstances throw things out of synch... I don't understand why it didn't work. Perhaps it only works within a certain nitrite range. I think I was up to 5.0 ppm this morning when I found this catastrophe. Which is insane considering I did a 50% change last night and Prime dosing.

I'm fairly certain it was nitrite poisoning. I had some fish such as my Jurupuri coming to the surface to breathe which never happens and I noticed most, not all the fish were breathing heavily. I was surprised though because neither the Swords or Corys were breathing with difficulty. I even saw a Khuli Loach make a dash for it.

I don't believe this was parasitic etc, because it was so sudden and the signs of Nitrite poisoning was there. And, it tested at 5.0 this morning which of course is excessive. I know that the amount of ammonia/nitrite a fish can handle typically varies from fish to fish. Some of these Cichlids do require very good water conditions. My Rainbow Cichlid survived and I know that they are typically considered to be very hardy.

Here's what I don't get though... I went home at lunch break today to do another water change and I tested the tank before the change and the Ammonia and Nitrite was at 0. How did that happen? I didn't have a chance to change the water before work when it read 5.0, so I moved as many fish as I could to another tank and dosed with Prime. So I don't see how I came home 4 hours later and got 0 readings? There's no way the bacteria could have caught up and eaten that much Nitrite that quickly is there?

When would it be safe to put fish back in that tank? How many days worth of consecutive 0 Ammonia/Nitrite readings would you feel comfortable moving some fish back in there? I'm flabbergasted as to what is going on.

*side note - I totally understand the value of a Q Tank, and wish I could set one up, however I don't have the option to at the moment as my free space is cramped. I buy my fish from the same LFS I've been using for a decade and I trust his stock. Have never had any serious health problems with any fish from there. But believe me if I could quarantine, I totally would. When I move to a new place soon, I will absolutely*