Newly Cycled Aquarium: Significant O2 Issues

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mark4785

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This weekend I purchased 5 Black Neon Tetras for a fully cycled open-top aquarium. Before going to purchase the fish I double checked that the ammonia, nitrite, PH and nitrate levels and temperature were all at suitable levels and they were all within range, as can be seen below:

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 15 ppm
PH: 7.2
Temperature: 27 degrees C

I introduced the fish to the aquarium at 2:30pm and by 7:00pm, two of them had died and the rest were gasping for oxygen at the water surface. I didn't think that I had any sufficient reason to check the water o2 levels since the water's surface is rippling quite a bit allowing oxygen to enter the water column. In addition, although this aquarium will be a planted tank at some point in the future, there is no co2 injection at this point so this couldn't be the source of low oxygen diffusion. At 7:30pm I checked the o2 levels using a basic test kit and got a o2 reading of between 1-2 mg/l. In contrast, my established planted aquarium had a o2 reading of 8 mg/l.

I am really confused and upset that I've lost two of these fish and would therefore appreciate it if anybody has any explanation as to why oxygen levels in this newly cycled aquarium are horrendously low.

Note: the remaining 3 black neon tetras have been transferred to my planted aquarium and are not gasping/dying.


Update:

Just thought I'd update interested reader's as to the o2 levels. It's not great news.

On the 16th of October I re-tested the o2 level and it was between 2 and 5 ppm using the Tetra O2 test. So it has improved slightly but when you consider that my co2 filled aquarium (to the left hand-side of this problem aquarium) has an o2 level of 8 ppm, you know there's something up.

There appears to be a white film in the water column (looks like loose skin) and it is also in the base of the filter system. Attached below are images of it:

dsc05662dm.jpg


I guess the only resolution to this is multiple 100% W/C's and lots of filter scrubbing (no chemicals and tap-water involved of course).

Any advice with regards to removing this skin-like floating substance as pictured?

Thanks.
 
That is a protein film caused by undissolved organics and natural oils from foods. It can be removed by 'wiping' with a paper towel and prevented with increased surface agitation.
 
That is a protein film caused by undissolved organics and natural oils from foods. It can be removed by 'wiping' with a paper towel and prevented with increased surface agitation.

Thanks for the reply.

What kind of foods are we talking about? This 'protein film' was here before any food had been placed in the aquarium.

Can you give an example of a undissolved organic? The tank water has only been in contact with pure ammonia, glass, silicone, rocks and play sand.
 

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