I'm not sure I explained myself correctly, so here's an "in other words"
version
Lets say you have a reef setup with a sump. You are growing Caulerpa or
macro alga in order to get nitrates down, caulerpa is a photosynthesizing
plant that needs NO3 and PO4 in order to produce the sugars that are needed
for survival. The caulerpa will only take up no3 as long as there is po4 in
the water column. Once all the nitrates get used up that leaves po4 in the
water column for all sorts of algae to thrive on (algae can survive on very
little no3). BGA will show up, but, if you ADDED more nitrates (say
10-15ppm) then the nutrient balance is restored and the bga will start to
dwindle away. this works vice versa with no3 & po4. You need to have the
nutrients in a specific range ALL THE TIME. For freshwater plant tanks the
perfect ratio for N&P would be 10:1. You need to focus on nutrient balance
in the water column and to grow good plants, don't focus on the algae. Give
the "ECOSYSTEM" what it needs and it will thrive. PO4 can cause BGA but not
directly, it will only cause BGA if there are no more nitrates!

I can imagine that alot of you run into this problem where you add a sump to
grow macro algaes in order to combat unwanted algaes in the display tank.
The macro algaes thrive in the sump for a while as there were nutrients in
the water column for it to feed on. But as the macro algaes grow and
nutrient uptake demand becomes higher eventually you will either run out of
po4 or no3, in which case its time to start adding those missing nutrients
back to the water column, unless you have a high fish load or do heavy
feedings. I suppose the optimal system would be a perfect balance between
fish poops and plant nutrient uptake. I think seldom would any of us have
this perfectly balanced system, it would need our divine intervention

For those of you with BGA problems, get your test kits out and see what the
ratios are for n & p in your setups. It will be interesting to see the
results. Dosing the water column with ferts would only be needed if the
balance between fishload, and fish feeding was out of whack, but this is
logical.
--
**So long, and thanks for all the fish!**


"Rod" wrote in message
...
Many belives, that exccesive PO4 causes cyano, and there are advices to
put DI filter after RO to reduce it (?)


that was a few years ago.. here is a post made by Eric borneman, after he
attended the NCRI.. th ethought is that cyano is not fueled by PO4. It is
heavily related to nitrates, temp salinity and current.. here is the link
(the
cyano part is about 2/3s down the page..
http://www.reefs.org/library/article/e_borneman4.html
Rod Buehler
www.asplashoflife.com