Re: CO2 loss

Hmm, the discussion is taking off. Great. However I'll try to split the 
subjects because the discussion is going in several directions.

CO2 loss: Let me summarize. This is a 5'x2'x2' tank with open top and a
4.5 gal TF in one corner of the aquarium. Underneath the aquarium, there
is a five chamber what we call 'biofilter with about 150 l (40 gal) of
water. The biofilter also has no cover. The water is returned to the tank
with an Eheim pump (forgot the model but it's rated at 38l per min and a
maximum pumping height of 3.1m). The flow from the pump is not reduced by
any means, just the water column of about 125 cm. The outflow of the pump
is about 10 cm below the aquarium water surface. This is just to give you
an idea of surface current. (Yes, I do have a big safety valve to prevent
backflow in case the pump stops).

 Water temp is 24 C or 75 F. 14 dGH, 10 dKH. NH4=0, NO2=0, NO3<10, Fe=0.2,
PO4=0 (all mg/l unless otherwise noted). The CO2 system is a Dupla setup
which I bought almost new from somebody else. He had been frustrated with
it because he bought it to reduce the pH in his 1000l cichlid tank (no
plants) to 6.5 at a KH of 10. His 500g CO2 bottle was gone in three days.
Almost new meant about half price for me :-). It's a pH controlled system
with the S type reactor. This reactor is driven by a dedicated small Eheim
pump. I don't think it wastes one single bubble of CO2 and it looks like
it's nowhere near its limits. I only bought another (1500g) CO2 bottle.
Right now the bubble count is about 4-5 bubbles/s and the valve is open,
well, more than 2/3 of the time. The pH is set to 7.3. 

So the high bubble count of CO2 really is going into the water. 

Looks like most of it is lost through the open surfaces in the tank and
filter. I also thought about minimal, but constant loss through the TF,
but even at a pH difference of 0.02 units (which I would have detected)
the daily loss would only ammount to about 16 grams of CO2. So that's not
so bad. (This is calculated with a pump rate of about 1000 liters/hour). I
am hoping for some significant CO2 production by the tank itself once all
the bacteria, fish and microorganisms get established. I would really hate
to put glass covers on the aquarium.  I guess I'll know in a few weeks (or
days?) how long the CO2 bottle lasted. 

Re: algae: As I said, this is a fairly densly planted tank with 2x150W MH
lamps (10 hours/day).  Phosphates zero and Nitrates low. Iron upper limit. 
Very low fish load. pH 7.3. I really don't know what's causing the algae.
Only two factors I can think of are too much light and too high pH. No 
direct sunlight.

Two questions are still out for grabs:
Is five crossocheilus siamensis not enough
anybody have any experience with making their own plant fertilizer (Fe 
EDTA and trace elements)?

Thanks everybody for their patience and help. This has been very 
interesting for me so far.