I didnt touch the filter media, except the top white filter, which needed replacing anyway. I dechlorinated the water, and the filter shouldnt have dried out in the 5 minutes it took me to change the gravel. I only removed a quarter of the water during the process. So you think my other fish should live through the cycling? I was worried it might be a parasite or toxin. Thanks for the quick reply.

Thank you for the suggestion about the hydrogen sulphide. That deffinatly sounds like a possibility. On a suggestion by the my local aquarium store I have added Tetra SafeStart this morning in order to help along the cycling process. My most recent water test results are as follows:

pH - 7.2

Amonia - 0

Nitrite - 0

Nitrate - 0

Is it normal for all bacteria to have dissapeared? Also dispite the lack of nitrite and amonia, the fish are still gasping for air at the surface... is this just part of the cycling process? I dont remember this happening when I started the tank.

Yeah I washed the gravel. Although it didnt seem to need much rinsing, as not much residue came off it. Also when i filled the tank it seemed clean.

I double checked the Nitrate reading because I thought I might have made a mistake. But got the same reading, 0. I honestly can't explain that...

Forgetting my manners btw,

Welcome to Fishcrazy! [wave]

Maybe not hydrogen sulphide then, that really does stink, you would have noticed it I'm sure. And odd that nitrite suddenly appeared but you never detected any ammonia throughout the time. What's the timescale of all this, how long ago was the gravel change?

Other things that occur to me;

-You added plants at the same time, I wonder if anything might have come in on them.

-A pH change of 0.6 over a short space of time is pretty significant, I wonder what caused that and whether it may have hurt the fish.

-The fish are still gasping at the surface suggesting amongst other things gill damage (ammonia may have caused this if there had been any) or there's low oxygen in the water. If you have an airpump it might be worth using with an airstone to see if that gives them relief.

What's your water temperature btw?

So everything has happened on a very short timescale indeed.

Rapid pH change, Nitrate gone from 20 to 5 to 0 in 2 days, grey scum/film on water surface.

Clutching at straws here really, but I wonder if there's some sort of chemical got in there which has reacted with nitrate to form an insoluble compound which floated on the surface and which is either toxic to fish or has removed oxygen from the water. I don't know of anything that would do that and never heard of anything similar happening but putting all the evidence together that's the best I can come up with.

Plants only produce oxygen in the presence of good light, in the dark they consume oxygen. In reality they'll actually make very little impact on the oxygen level in the water. If you reposition your filter so that the output is causing as much agitation at the surface as possible this should help oxygenation. I guess that the surface isn't being agitated much if a film has been able to form.

Very good news, just a pity you had to end up losing fish in the process. Must have been low oxygen that was affecting the fish, seems like there wasn't anything that caused lasting damage? Still bamboozled by the drop in nitrate though.

At least it looks like you don't have to worry about going away for a week any more.

Could be right about the oxygen if the lid was effectively sealed - but remember fish will stay alive in the plastic bag you buy them in for quite a long time with a tiny airgap at the top, and fish are shipped in bags like that from the other side of the world (albeit with pure oxygen rather than air) so I'd think he lid would have had to have been sealed for a long time before it would result in oxygen starvation.

I use airstones in all my planted tanks because I use air driven undergravel filters. The plants are fine, just like those you see growing in river pools below waterfalls. There's no good reason I can see that you shouldn't run an airstone 24 hours, and at least one good reason why you should!