AAAAAH Bad Water Change...

A few hours ago I did a water change on my 20G tank, just like I do every week. I syphoned off 30% and slowly bucketed in water over about an hour, the new water had almost identical pH and temperature. I consider my water supply to be very good (I live in a rural area and our water is sourced from mountain creeks). I don't treat the water with anything because the chlorine levels are low. No chloramine is used here.

About an hour after the water change most of my fish started acting a bit wierd; hovering at the top, breathing a bit faster and generally acting strange. Then one of my neon tetras died and now it looks like the others will follow. The Neons have red gills, breathing faster and are cycling from going belly up then righting themselves then belly up again etc. They don't have any physical damage.

After water change:
Ammonia - 0
Nitrite - 0
Nitrate 10ppm (normal)
pH - 6.8
Temperature - 25c 76f

It looks like ammonia poisoning but it's definately not that. The only thing I can think of is copper poisoning, but I didn't use any hot water from the tap.

Anyone got any ideas as to what it could be? Thanks.

That's what I thought at first, it's possible but I highly doubt it. The tap water doesn't smell or taste like there is an increase in chlorine. Plus the council only put extra chlorine in after heavy rain, and it's pretty dry at the moment. Plus if it was chlorine I'd notice an increase in ammonia due to the bacteria being killed off by the chlorine, ammonia levels are still 0ppm.

Update:
I put a bubbler in the tank just incase it's low oxygen levels.
Another Neon Tetra has died and another is looking pretty close to dying.
Some of the fish are now glancing/flashing off things as though their itchy. The lone Danio has managed to rip one of it's eyes out too, I think it rubbed up against something sharp. What I don't know.

Not a good day...not a good day at all .

I'm going to check all the parameters again soon, just to make sure it's not ammonia or nitrite. I just remembered; my sister did a water change this morning, same bucket same tap same water etc. Everything is fine in her tank.

It's so annoying not knowing what's wrong.

I don't think so. I have my own "Aquarium Buckets" which are only used for water changes and are kept upside down and out of the way.

I've now lost 3 Neon Tetras, only one remaining. The other inhabitants are okay but they still don't look 100%.

I lost all 4 of my neons, poor little guys. My one-eyed danio seems fine and is eating normally. All my other are fine now and acting like nothing happened. I was sure I'd lose my Otocinclus Catfish but they didn't even act like something was wrong.

The only reason I can think why it happened was that there was a slight increase in the chlorine levels. It must have been tiny though because any noticable increase would have wiped out the nitrifying bacteria before the fish.

I didn't think neons were so sensitive, a slight change and they drop like flies. I'm going to let the water sit overnight now, just to be sure.