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We have a 500 gallon system that had been fully cycled/established for a few months without fish.  We added tilapia fingerlings last week.  They have been doing great, eating like crazy.  This am there is a nitrite spike........ph 7.6 (normal for this system) ammonia 2 (little high for this system)  nitrites 5+ and nitrates 80 .  During this week, while adding water to system, it was left on too long  and there was probably about 50% water change.  That happened about 4 days ago which was about 3 days after the fish were added.  My question now is how should I address the spike ?  Do I change out some water ? Add Salt ?  Do nothing ?  The fish seemed fine this morning and until I read the forum, did not realize I should stop feeding them......what else ?  thanks

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Just stop feeding them for a couple of days. Tilapia are tough fish. The only thing I've found that can kill them is to shut off their air. I assume the ammonia is going up as well but since you have nitrates your system is cycling and will break down the nitrites. When I first started I worried all the time about the #s but in the end pH and ammonia are the only #s that should get your interest. If your ammonia isn't climbing you're good. pH is the backbone of your system.

thanks jeff !

You're not kidding tilapia are tough.  They'll even survive a low oxygen condition that would kill most fish.  How much nitrite they can survive, I don't know.  Salt wouldn't hurt.

My last low oxygen episode I lost 47 lbs of Tilapia but they were all the larger fish. Every one 8-10" and smaller survived.

that must have been painful

Jeff S said:

My last low oxygen episode I lost 47 lbs of Tilapia but they were all the larger fish. Every one 8-10" and smaller survived.

Yes especially since the weather and time availability kept me from even burying them in a garden. The kind of thing you can't keep laying around.

Jeff......still no change in nitrite after 4 full days of not feeding.  It is possible it has changed but my test kit only tests to 5 and color is red violet still.  Did not add salt or do water change.......what do you think ?  Continue with no feed ?  The fish seem fine.......active and acting hungry.

Looks like a water change might be in order. What are your ammonia and nitrate #s? I'm no expert but if ammonia is under control and nitrates are rising the nitrites should come down. There are 2 types of bacteria involved in the break down of ammonia but I can't see why you would have one and not the other. I would think the over exchange of water you did would have washed away everything except the bacteria. Where did the water come from?

ammonia  is .50 and nitrates are 40 -80.  The water is from our well.  Before the unintentional water exchange and before we added fish, I did not add ammonia on a regular basis for a couple weeks since  it was testing that there was a small amount of ammonia and I did not think I needed it but I may have killed off some bacteria during that time.....that is the only thing I can think of that could be contributing to the high nitrites.

How many fish do you have? Maybe the easiest thing would be to put them in a smaller container that you can easily do water changes to keep the ammonia and nitrites down until your system has for sure cycled. Otherwise you'll worry yourself sick about the fish or worse loose the fish. I know when I started had 1 male and four females and was constantly worried until they reproduced. Even then I separated fish into a separate aquarium so if something went wrong I wouldn't loose them all.I'm running 3 separate systems now but thinking of consolidating. 

The bacteria which consume nitrite establish after the ammonia nitrifiers and they reproduce more slowly.  It shouldn't take much longer and you only go through this once.

Jeff S said:

There are 2 types of bacteria involved in the break down of ammonia but I can't see why you would have one and not the other. 


So would you recommend not feeding them for a few more days or take the fish out until the nitrites drop ?
George said:

The bacteria which consume nitrite establish after the ammonia nitrifiers and they reproduce more slowly.  It shouldn't take much longer and you only go through this once.

Jeff S said:

There are 2 types of bacteria involved in the break down of ammonia but I can't see why you would have one and not the other. 

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