Low oxygen levels

[Tony Roberts]

Tank parameters:
Trigon 350 [ set up 2 months ago] c 270 litres of water plus wood and plants [ we had duck weed for a while but have taken all that out to allow full open water at the surface.
Dennerle substrate
Dennerle recommended list of plant additives etc followed.
Water change programme of 25% fortnightly. Tap water adjusted to tank levels before being put in
C02 diffuser plus indicator [turned off at night]
Water temp 28C
PH 6.8
KH 3-5
CH 10-13
Nitrite levels negligible
Small amount of nitrate
Juwel filter sponges cleaned as per Juwel recommendations
Regular testing for PH NO2 KH CH
Mixed population of 8 small discus, 10 neons,4 bronze cats, 10 ottos, 10 pygmy cats, 2 small gouramis,
1 ancistrus.
All fish have fed well from the start : Flake, Prima ; frozen bloodworm and discus mixtures plus regular brine shrimp and occasional daphnia.
One feature has been relatively rapid breathing by the neons in particular but otherwise the tank has settled very well; readings are constant; fish are feeding and interacting well. Plants are growing well though not as profusely as the Dennerle literature would suggest!

Late last night the breathing rates of the smaller fish went right up and all the larger fish went up to the surface. Basic tests for ph, NO2 and KH done and were satisfactory. 16% water change done . Another 16% water bchange done early next morning. Fish returned to normal patterns for an hour or so but some are now returning to the surface [not all but most have fed ]

O2 test has shown 2mg per litre which looks very low.
We have adjusted the pump outlet to agitate the surface better and kept the CO2 off for the moment. The carbon sponge is in to take out any toxins and a treatment used recently for bacteria control.

Any ideas please on the immediate problem and any other reasons why the oxygen levels appear to be so low?

 

[Devilishturtles]

I don't know about the O2 levels, but just to rule this out, what is the ammonia level in the tank? You set it up 2 months ago? Did you cycle it? When did you add the fish? I don't know those names, since they aren't supplied in the US, but what kind of filtration do you have, and how many GPH is it?

 

[jsoong]

An O2 test??? I have never heard of a home O2 test kit (but I may be ignorant) ... FAIK , you need electronic gizmos to measure O2 (lasers, mass spec) .... and O2 level should be in the range of pO2 of 140 mmHg or about 5 ml per l.
EDIT: That works out to be 8mg/l <not g/l as prev. posted - wrong decimal places in head when coverting to unfamiliar units > ....

I agree with Devilish, you need to check your ammonia level ... your tank might still be cycling.

Also ... what is negligable NO2 level? Any NO2 is not acceptable in an established tank .... 0.1 ppm or more will stress fish. What test kit you are using - if you have the stripes, they are unreliable on levels and you should be concerned if ANY NO2 is detected.

NO2 posioning causes met-hemoglobinemia, which blocks O2 uptake. Fish will exihibit same symptoms as O2 starvation. NO2 posioning is much more likely than O2 depletion. Since your pwc appears to improve things, it makes me think that the problem is either NH3 or NO2.

BTW, your KH/pH looks too low for safe CO2 injection (if that is the level without added CO2) ... If that is actually with the CO2 in, then it is fine.

 

[Tony Roberts]

Thanks for the ideas. We have made real improvements by redirecting the outlet from the Juwel 1000 pump which has rally agitated the surface of the water. The fish very quickly breathed less fast and came dwon from the surface. This has continued and the breathing is now much slower.
An ammonia test gave no reading at all i.e. no ammonia.
The lowest indicator on the Tetra NO2 test is >0.3 and the tanbk has never had a reading higher than this.
The kh/ph reading is with CO2 injection
Tetratest do a simple but good O2 test which I now use weekly.