They're Dying ... Help!

[EasternCanuck]

Hi there,

I'm not a total noob at freshwater fish keeping but it has been a long time since I had my old 33 gallon set up back in Canada.

I live in Singapore at the moment and since I am living in a tropical region I figured I would try my hand at fish keeping again (it's much cheaper here than I remember it being back in Canada).

I just set up a new 20 gallon tank, purchased from one of the many aquarium shops on my street. I have it planted with 8 different plants, have a piece of treated driftwood in it, and have a soil substrate (little brown balls of dirt I guess? I never had this in Canada but it is common here and I am told it is good for plants).

I have no canopy but have a double light on top (12000K each light) and the lights are on for about 12 hours per day (probably too much).

The filter is an in-tank filter (comes as a kit, Juwel Rekord 600), which I have never used before (I had an aquafilter before, which is like a little waterfall).

One problem here is temperature and I have a little fan that blows over the water surface to help keep it between 26-28 Celsius.

I have a CO2 tank hooked up with a diffuser at a rate of one bubble per second (the little bubble counter). I have never used a CO2 tank before but they are common here (they weren't back home).

I hope that gives a better picture of my tank. My PH is below 6 at the moment, but I am attributing that to the CO2, driftwood and soil substrate.

At the advice of my local aquarium shop, I put in 8 cardinal tetras and 4 Rams 2 days ago.

I was getting hair algae so I added in about 40 little shrimp (Japanese name I can't remember). Again at the advice of my local aquarium shop.

It all seemed fine until today when I came home and all my fish were swimming at the top of the water? My water seemed a bit cloudy as well.

I did a 50% water change as nitrate/nitrite levels were very high and it seemed there was no oxygen in the water. I have lost 4 of the 8 tetras and I am fearful for my Rams.

Even with the 50% water change the tetras are still at the top of the water. The Rams are hiding.

I have turned off the CO2.

Help?