New Kribensis Very Inactive

[lifeofbrian]

I bought a male and female kribensis about 2 weeks ago, and they have been great, very active, and now all of a sudden, today they are both just near the bottom of different parts the tank with their mouths opening and closing rapidly and not eating. I tested the water: Ammonia 0, Nitrite 0.5, Nitrate 15, ph 6.4, KH 100, GH 250, temp is 24c.All other fish are fine, Otto, Cory, Tetra and Rasboras.
 
This is their tank.

 

[gmc1]

Your nitrite should read zero.i would do a large water change.

 

[Akasha72]

agreeing with gmc. The Nitrite is too high. If you zero it they should perk up. Once you've zero'd it keep an eye on it for a few days as this may happen again. 
 
How long has this tank been running and have you cleaned the filter recently?

 

[lifeofbrian]

OK, just did a 60% water change. I have aqualibrium salt, but have never used it before, is it worth dosing a small amount? Oh and yes, just did the filter last week.

 

[Akasha72]

if you cleaned your filter recently this can be the reason for a small spike. With all the best will in the world and all the best advice it's still possible to over clean. We've all done it. This is why I now run two filters and clean them alternately. It prevents that 'spike' if I over clean
 
Keep an eye on it for a few day and the filter should catch back up
 
Don't add the salt. You don't need it in my opinion. Just cut down the feeding and test the water daily and water change if ammonia or nitrite are over zero.

 

[lifeofbrian]

Well, I lost two Kribensis and one Otto in 24 hours, all my other 17 fish are fine, some of them are over 4 years old. What the hell happened? All the fish that died I bought two weeks ago and seemed fine and active for all that time. I just don't know...

 

[Akasha72]

some fish are more sensitive to water problems than others. As upsetting as it is to lose the fish it will have taken some strain off the fiiter. All the same I'm sorry to hear you lost them