All Fish Dead! What Happened?!

[ObsidianMan]

Bit of a disaster in the fish tank.  I say all the fish are dead, but there are two danios remaining.
 
Tank details:
 
200L Tank
Ammonia:  0.0
Nitrite:  0.0
Nitrate:  40
Ph:  7.4
Temperature: 28C
 
Filter: Fluval 405
 
Fish:
Angels x2
Glowlight Tetra x10
Mollys x2
Danios x2
 
What happened:  I just went to feed to fish in the upstairs room (first time I have been in there today), and I noticed all the fish except the danios were dead.  My first thought was ammonia, but after doing all the tests the water checked out fine.
 
Two weeks ago my old filter in the tank packed in, and I had to replace it with a spare Fluval 405.  I also took the opportunity to put in some new fake plants and 3 new ornaments all brought from Pets at Home.  All of this was thoroughly cleaned with de-chlorinated plain water before going into the tank.  These have been in for 2 weeks with no signs of negatively affecting the tank or fish.
 
For some reason the old media didn't fully take in the Fluval 405 and the next day I had a huge ammonia spike of 1.5. (A brisltenose and a molly died during this). I did a large water change and put a x5 dose of Seachem Prime in the tank.  I took some media from my other tank and added that to the Fluval 405.  By the next morning the water parameters were stable again.  The water parameters have been fine since then which is two whole weeks.
 
So I'm at a bit of a lose as to why the fish died today.  Part of me thinks it might be the heat - I found the fish at 21:00 I've no idea when they died, I haven't checked on them since I fed them last night, when everything seemed fine.  It's possible that the temperature in the tank hit 30C during the day, but I would have thought the fish could cope with that...even though it is a bit high.
 
Anyone have any ideas as to what could have gone wrong?

EDIT:  I forgot to add that the remaining danios are breathing at the surface of the tank.

 

[fluttermoth]

It could well have been related to the hot weather; most of the fish that you've lost don't like it too hot (although danios are very tough, which is probably a couple of those made it), although I am surprised at the angels.

It probably won't have been the temperature itself, but warmer holds a lot less oxygen so, in the absence of anything else, I would suspect that.

I know it's not of much help to you now, but for the future and anyone else; increase the aeration as much as you can; add an air stone, if you have an air pump, or raise the filter outake (or lower the water level). Keep the lights off during the day. Take the hood/lid off, if you can and you don't have anything that'll jump, although you can use just condensation trays if you do.

 

[ObsidianMan]

Thanks for the quick reply.  I did wonder about lack of air.
 
The filter outlet is quite close to the surface and it is causing the water to 'stir' quite rapidly at the surface...but it's possible that due to the heat it isn't enough.
 
One thing I forgot to mention in my post is that the danios are breathing at the surface - so it may well be an oxygen issue.  Seems very sudden for all those fish to die with much warning otherwise and lack of oxygen could explain that.
 
I want to replace the fish as soon as possible - as I don't want a media die-off.   But I don't want to put new fish in without knowing the cause.  I have an air stone and pump so I will add that in.
 
Any other thoughts or suggestions on what else may have caused this?

 

[star4]

Ditto FM, the angels should have been ok at that temperature. Do you have an air pump running? where is your outlet on your filter situated? if it is low in the water raise it up so you get more oxygen in the water. 

 

[ObsidianMan]

Outlet is near the surface - there is a lot of water agitation there.  But I don't have an airpump running in the tank.

 

[star4]

If the danios are gasping at the surface and all the other fish are gone, if it was low oxygen the danios should have recovered by now or started to at least.
 
Have any chemicals been sprayed at all, fly killer, hairspray etc as a precaution I would remove the new decor (not that it is likely but if nothing has been sprayed then remove them) and do a water change now, see if there is any improvement in the danio's after.

 

[ObsidianMan]

If the danios are gasping at the surface and all the other fish are gone, if it was low oxygen the danios should have recovered by now or started to at least.
 
Have any chemicals been sprayed at all, fly killer, hairspray etc as a precaution I would remove the new decor (not that it is likely but if nothing has been sprayed then remove them) and do a water change now, see if there is any improvement in the danio's after.

 
Thanks.  I just put the airstone in the tank and as I did so one of the danios died right there on the spot.  Only one remaining now.
 
I will remove the decor and do a water change.  Not sure if there is any chance of saving the remaining danio now though...and I don't have another tank to put it in.
 
No sprays have been used in or near the room.  So I don't think it is that.

 

[tunagirll]

You can even put the danio in a bucket if you need to Sorry about your fish! Is your test kit okay? I would keep a sample and get it checked at the lfs because it sounds like ammonia/nitrite poisoning.

 

[nortonmad213]

your levels may of gone above the test range that your set can handle. if you dilute some tank water with deionised water in a test tube you may get a different reading letting you know possibly

 

[gh74]

How long have you had the fish? any fish i bought from P@H all died within weeks of having them in the tank...poor quality fish...there is a place a few hundred yards from me that supply LFS & P@H and some of the guys work in it wont even buy there fish so that say a lot,search for somewhere that buys there fish in directly from south america,asia etc,so i would say that was one off the reasons they died and the other not enough oxygen in the tank.even if you know of any breeders buy from them ....just my thoughts as ive had all the probs in the past