2 of my fish died suddenly with no symptoms

Question
Hi,
Last night my zebra danio died. I noticed my 7 fish and 3 apple snails swimming near the top and the zebra was swimming upside down. It was trying to right itself but couldn't and eventually it stopped breathing and died. This only took about 5-10 minutes. It had no other symptoms apart from floating upside down. I did a massive water change (about 75%) and the rest of the fish seemed to perk up a bit. I also did loads of tests - ph, ammonia, nitrate, iron etc. and all were fine. I have 2 danios left, 1 white cloud minnow and 3 guppys. But then this morning, the minnow was upside down too....dead...again with no other visible symptoms. I changed the water again and did more tests, but again the results came back perfect.

My boyfriend and i have had a tank for at least 6 months and none of our fish have ever died. We had always kept coldwater fish, but then a couple of weeks ago we bought a bigger tank (7 gallons), then a week later we bought the 3 guppys - 1 male and 2 female. We also got a bigger filter (fluval 2), a heater and a thermometer so we could turn it into a tropical tank. The fish have all been fine in this new tank - or so we thought...
Do you have any ideas on why the 2 fish died suddenly?  Could it be something to do with lack of oxygen or maybe neon tetra disease or something like that?  I took a sample of the water to the petshop today and they did ammonia tests which turned out fine so it isn't the water...


Answer
Hi Gayle
Sorry for the problems you're having.   

I can think of a few possibilities of what's happened, but to say exactly what happened is kind of hard.

My first thought is the tank is a tad overstocked.  And, it's a new tank with new filter etc. so it most likely went through a cycle process.  You said you checked the water parameters and they were fine, but it seems from what you said, you checked them after doing the large water change-not sure if that's the case, but if so, the water change would've given the good readings on the test because it diluted the toxins in the water.  And, the fact that the fish perked up a bit after the water change, kind of leads me to think the water quality wasn't good before the water change-which is common in new tanks.  

Also, a lot of the test kits and some pet stores will say a small amount of ammonia or nitrites is safe.  That's not the case at all, any amount of ammonia or nitrites is dangerous and stresses/kills fish.  

Now, you also just added the new guppies to the tank.  They may have brought a disease/bacteria into the tank with them.  I don't think it sounds like neon tetra disease, there would be other symptoms showing.  With the lack of oxygen, if the filter is agitating the water surface good, then there should be adequate oxygen in the water.  The surface agitation is what allows the gasses to exchange, and oxygenate the water.

Another thing with those types of fish-ones that are very easily bred like livebearers, danios, etc., there's a lot of poor quality breeding and inbreeding-like with dogs and other animals-going on.  The fish now are just not as hardy because of that, they're breeding the good qualities out of the fish.  

So, I think it's kind of a combination of all I mentioned above, mostly with the water quality and the new tank.  Also, you'll be better off doing small daily water changes(if there's ammonia/nitrites present) as opposed to one large water change.  Doing a large water change will most likely alter the ph and the temperature which can stress the fish.  If the ammonia and nitrites are at 0 ppm, and nitrates under 20 ppm, a weekly water change of about 25% with a good gravel vacuum is usually what's recommended to keep the water parameters in check.

Hope that helps, and let me know if you have more questions.

Christy