Nitrite Spike

[TheSims3Dude]

Hello, so I've come across a problem, I have a 20 gallon tank that's been set up for about 8 years at least, I have about 7 fish in there, (well did) I purchased some at PetSmart about a week and a half ago, 4 guppies, apparently they didn't bother telling me that the water level would spike, I've had 3-4 fish since then die due to Nitrite levels, (or Nitrate) not sure which one they said, I did a 10-15% water change right now, and there's 3-4 fish in the tank still, they seem to be doing okay, the one molly is acting aggressive lately, (female) I'm not sure if that's just her, or if it's from the spiked levels, I also put some Ammonia, Nitrite/Nitrate remover in the tank as directed, will that help it? How often should I be doing a water change?
I have a test kit but as they never told me how much my levels were, I don't know what to look for, what's the "safe zone"? Any help or suggestions would be greatly appreciated,

 

[This Old Spouse]

It would be your nitrite levels being too high that would be a problem here, I believe. Your ammonia levels are most likely elevated as well. Your ammonia and nitrite levels should both be at 0. Your nitrate can be up to about 40 ppm with no problems (although that number is argued by lots of folks).

You need an API or other professional liquid test kit. I'd be doing about a 50% water change again tomorrow, then check your levels. If the ammonia and nitrite are still high, do another water change.

I used Stress Coat when I do changes because I have a bit of ammonia in my tap water and it neutralizes it.

 

[TheSims3Dude]

Oh! I have an API test kit (the liquid one) I also have stress coat so I'll put some of that in too Thanks, should I check the levels tonight?

 

[This Old Spouse]

Sure, check the levels to see where you're at. Always a good idea!

 

[Moley.]

I believe Nitrite is a bigger killer than Ammonia, the unexpected Ammonia load on the filter Bacs has resulted in a Nitrite spike, although a short loved one, but its enough to knock off weaker or more sensitive fish.

Keep testing and doing water changes until the mini cycle ism over, then give it a few days of spot on water parameters before slowly adding more.