Sudden fish deaths - what do we think happened?

[Katherine]

I've copied and pasted this from my Kids' Tank journal to see if I can get some more eyes on it. Background that's not in this post:

The kids and I set up a 10g tank on February 16th. The filter is an ACO sponge that's been in my main tank for about 4 months. A few days after setup we moved 2 mystery snails over from my tank to ensure the sponge could handle a light feeding. Ammonia and Nitrite had consistently been 0 for a week before adding 5 neon tetras early this afternoon.

....................

Well, I have sad news. We lost all 5 of the neon tetras. Let me walk through what I think happened and see if you guys agree with my assessment and what I can do from here.

We temperature acclimated by floating the bag in the tank for 20 minutes, then mixed tank water into their bag, roughly doubling the water volume over 15 minutes or so. Dumped the bag over a bucket into a net and released the fish into the tank

Immediately on putting the fish in the tank they seemed fine. Swimming around, exploring.

Maybe half an hour later I noticed them hanging out at the surface a little bit. Not obviously gasping, but it seemed a little strange. I tested the water then.

Temp was 74.something

Ammonia 0

Nitrates 10ppm

Nitrite 0

GH over 300ppm

KH 40ppm

PH 6.4

No chlorine - my tap also tests as no chlorine, but with .5ppm ammonia, so I think that means they use chloramines instead

At this point I did a 50% water change (as close to the same temp as possible, tap parameters match tank parameters besides having .5ppm ammonia) and removed the lobster decoration and smelled it (it smells funky wet, I did not notice this when setting up the tank). I think it likely the issue was chemicals coming off the lobster.

We lost one of them just after the water change. I think this was from me putting the dechlorinator in the tank instead of in the bucket before adding the water. I remembered after this first fish passed (of course!) that I used to have issues doing it that way with danios of a similar size to these tetras.

The others were continuing to spend time at the surface, still not all the time, and not obviously breathing heavily.

At this point we had somewhere to be and I didn't have time to attempt anything else, and wasn't sure what else to attempt even if I did have time.

The other 4 were all dead by the time I got back.

I have moved the 2 mystery snails back to the 75g tank, and plan to do as close to a 100% water change as I can every day for a week or so before putting any livestock back in this tank.

Is there anything else I should be doing? Is it probable that the lobster wasn't the cause? The pink plastic plant has no smell. I've used the barrel decoration before with no issues, the green thing is a hunk of colored glass, from the rock section of one of our LFS so I wouldn't think it would cause issues, but then again, the lobster was also from the same LFS.

[Katherine]

I will add, now that I've done a 90%ish water change with the lobster removed, the bubbles coming from my air stone are larger. I had thought they were small before because of how I'd tightened it. It's one of those with the felt disks from ACO.

[gardenman]

Neons can be a bit fragile. I don't see it being an ammonia issue. Realistically, five neons in ten gallons aren't putting out clouds of ammonia in a quantity that would likely cause an issue. The low-ish pH makes ammonia less toxic also. At 6.4 pH you'd need a fair amount of ammonia to be toxic. I doubt that it's a "normal" tank decoration sold for aquarium use that's causing the issue. The snails were alive and well, so had the water been horrible they'd have likely had an issue in the week they were there. You never know what's been going on in the retailer's tank. He/she could have been experiencing large daily losses also.

Oxygen deprivation is typically why fish hang out at the surface. Either an issue with their gills or insufficient oxygen in the water. Ammonia burns can cause gill issues and it's possible that was damage done before you got them. A tank with any type of water movement in any quantity should stay oxygenated enough for neons. They're small fish with limited demand for oxygen. 

I don't see a smoking gun here and would assume it was one of those things that happen from time to time.