I've done it again...

[Dwight]

Last week I posted about cycling issues I was having, and after getting some good info, I went back and checked and saw the parameters have been 0, 0, about 40 and pH 7.0 nearly two weeks. Again, thinking my tank was cycled, we went to a fish store (not Big Box Pets) Saturday and bought six Zebra Danios and thee Mystery Snails for our 20 gallon tank. We also got a couple live plants. There were no other fish in the tank and a little algae.

Things seemed to be going well but when I came home today, I noticed that rotten egg smell and saw the fish were not as active as they have been, lethargic. I tested the water and still had 0 Ammonia, but Nitrites had spiked to over 1.5 and Nitrates were near 140.

I did a 15% water change and the Danios immediately swam to the surface, the smell intensified and within ten minutes four were floating belly up and the other two were struggling. I did manage to get those two of into the 10 gallon but they don't look like they'll make it. I'm not sure where the parameters are there, but leaving them in the 20 would not work based on the other four.

What have I done this time? And why the spike in egg smell so quickly?

 

[JettsPapa]

I don't know why it spiked, but with nitrites that high a 15% water change didn't do much good.  I'd recommend at least 50%, to get it down below 1 ppm, and then another one tomorrow.

By the way, the rotten egg smell is usually from sulfur, so I don't know why you'd be smelling it.  On second thought, I believe Prime, and probably other similar products, contain sulfur.

[Dwight]

Ahhh...that could be the sulphur smell source. I use Prime and API to detox my tap water. But this had sat since Friday last week and this smell literally "exploded" as soon as I added the water.

[Flumpweesel]

Give your substrate a stir or a really good vac, I think you can end up with pockets in the gravel of a new set up that have the wrong bacteria especially if it's a deep substrate. 

Adding the snails and plants could have disturbed something and damaged your cycle. 

This comes from a very old memory of advice given when I was having tank issues. But a good clean and big water change is probably good idea right now.

Fingers crossed for your danios.

[Drenon88]

First off, eliminate Prime from the equation until you figure out what is going on.

Your nitrates climbed from 40 to 140 in approximately 3-1/2 days? To reiterate what JettsPapa said, to combat a nitrate spike like that, you must change a lot more water than you did. Next time, change 50% of the water and be prepared to change more water 24 hours later after you've tested for nitrates again ... until everything stabilizes.

What kind of filtration do you have on the 20g tank?

Edited November 17, 2021 by Drenon88

[HH Morant]

140 ppm nitrate does not kill fish.

Nitrite is the problem. The bacteria that converts ammonia to nitrite sometimes develops faster than the bacteria that converts nitrite to nitrate. So the ammonia gets converted to nitrite and then the nitrite builds up.

I agree that big water changes are the best way to cope until the bacteria fully develops. I routinely do two thirds (67%) water changes - even when nothing is wrong. 

You have nitrates, so you know the bacteria to convert nitrite to nitrate is there. It just needs some time to grow.

[Dwight]

@Drenon88I'm using Aqueon Quiet Flow 30 on the 20 gallon tank and a Quiet Flow 20 on the 10 and a small sponge in the corner of both.

@Flumpweesel I've done occasional gravel vacs getting to this point, and that has worked well...

After thinking through this last night and this morning, I believe I caused this in a panic. overfeeding can cause Ammonia spikes (which I likely missed Monday), the ammonia converted to Nitrite (which I caught Tuesday). I came to virtually the same conclusion as @HH Morant

I managed to save just one of the six by moving it quickly to the 10 gallon. As of this morning, it was fine, the snails are fine still in the 20 gallon and the bacteria bloom that popped up during my panic last night in the 20 gallon tank has subsided quite a bit this morning.

I'll continue to monitor and tamp down my reaction a bit next time...

Thanks for the replies!!!