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CO2 reactors queries and hypoxia

> >The excess CO2 accumulated between the water surface and the cover glass
> >until all the O2 had been displaced and the fish asphixiated.  You DO have
> >to worry about too much CO2.  Unless like George you do not use a cover
> >glass (am I right there George?).
> You are right. The CO2 will sit on top of the water. It is a heavy gas
> compared to the air nitrogen oxygen mix. 

Actualy I would have said that you could still get into problems if the 
rate of dissolution with the jar full is greater than the use/loss of 
CO2. This may happen especially if you have a powerhead etc under the bell.
> I have a similar one made by Dupla. The 3 ranges in mine are low(blue), OK
> (green) and high (yellow).  You are supposed to change the indicator

> All you need is a small gap in the cover glass to fix that.  Is there any
>  reason other than water evaporation to have a cover?  If not, you don't
> have to seal it to contain the majority of the evaporation.

There is an inconsistancy here, on the one hand you say you only need a 
little gap to stop asphyxiation and on the other you say that you do not 
need to seal the container to contain the majority of the water loss. To 
my mind if both are gases escaping from the aquarium they will do so at 
nearly the same rate and the rate at which oxygen enters will be similar 
to the rate of loss.

I think that the problem is getting a bit confused, water can only 
dissolve the gases that is comes into contact with. By adding CO2 we are 
increasing the relative content of CO2 in the "air" and in that way you will 
get a slight displacement of the other gases from the water. However the 
plants produce a much greater amount of O2 under these conditions and so 
the partial pressure of O2 increases and thus restores the balance somewhat. 

There are two factors that may be the real cause of the loss of fish 
under these circumstances. The first is pH, I had the misfortune to have 
a solenoid stick on with my pH controller. The next morning the pH was 
less than 5 and the fish nearly dead. I imagine that the fish were 
definetly pH shocked. The other is CO2 concentration, haemoglobin and 
other oxygen carrying molecules can bind CO2, this is not normally a 
problem but under these situations (extreme CO2) it will compete for the 
oxygen binding sites and reduce the available supply of oxygen to the 
fish. Added to this you will also have reduced the amount of O2 in the 
water because of the massive change in CO2 displacing it from the water. 

Added to this is that I imagine that the water/air layer under the cover 
glass is at a slight positive pressure relative to the air. If the room 
is cooler than the tank (the case generaly at night) the warm humid air 
above the tank will escape slowly and this will limit the oxygen that can 
diffuse into the tank. This would probably only work for a small gap, but 
then again I could be wrong on that.


Somebody I forget who said that they thought that hypoxia was a 
relatively rare condition in planted tanks. I have had it a couple of 
times and I thought I may put in my two cents worth.

A friend of mine replanted and already planted tank which he wanted to re 
decorate. That resulted in very murky water and the next day nearly all 
of his fish died. I imagine that the organics released from the replant 
drasticaly increased the oxygen demand in the system and killed the fish. 
There was no ammonia.

Another friend of mine had a fiendish attack of cyanobacteria, it was a 
sticky clumpy mess that stuck to everything in addition to being carried 
in the water. On two occasions when this stuff was at its worst he lost 
all his fish. There was again no ammonia or nitrite, however all the 
sword plants had a black ring around their leaf petioles that was not 
there the night before. I think that the black ring is the result of 
local annoxia due to the lack of oxygen in the water. Plants are better 
able to deal with oxygen lack than fish because of their increased 
ability to produce alcohol anaerobicaly. No treatments were in the tank 
at the time erythromycin did not treat it at 5mg/l. So the person that 
recomended that erythromycin should be used at very low doses should take 
note of that. It was eventualy killed by turning the lights off for 3 
days and has not come back since.

Finally I have had hypoxia when I changed filter media. If I change all 
the filter media that I have in the planted tank in one go the next 
morning the fish are at the surface gasping and all the swords are ringed 
with black on the petioles. No it was not ammonia or nitrite as both 
registered zero on the test kit (and the test kits work). I imagine that 
the presence of free ammonia meant that the plants metabolic activity 
that night was much greater leading to the annoxia. It also occurs on 
night two and three but not as severe.

Hope this helps some people sorry for the long post.

Peter Hughes