"Mark Trueman" wrote in message
...
Firstly some background

Im running a 160litre planted tank, pretty heavily planted. Pressurised
co2
at about 25 ppm, 120 watts of lighting 10 hrs a day; 5hrs light, 2 dark, 5
light. Plants are growing ok but i think there is some problem with the
take
up of some nutrients. Im dosing as recommended by numerous sources as
follows. Firstly, a rundown of my tapwater, im doing 50% water changes
weekly on a saturday.

There is no need to have a dark period in your tank. It disrupts the natural
daily plant cycle and it doesn't help with algae.

Tap water
Iron 0.017ppm
Nitrate 40ppm
Alkalinity as HCO3 289ppm
Calcium 134ppm
Magnesium 3.8ppm (*LOW??*)

I would add some Mg in this case. It might never be depleted at this level,
but I believe it has to be balanced with the calcium, or the calcium might
block the consumption of Mg. I'd aim for 50 ppm Mg. MgSO4 is cheap and easy
to find. I have a lot of Ca but no measurable Mg in my tap water, and I add
about 2 degrees worth of Mg every week.

PH with co2 injection in my tank is about 7.2

According to my calculations, your KH as CaCO3 is 26 degrees, which would
give you 49 ppm CO2 with that pH. If that's correct, it's a little high.

Here is what i have been dosing....
20ppm potassium sulphate once a week with water change
0.5 ppm potassium phosphate twice a week, on a tuesday and thursday
No nitrate as yet, still high in my tank.
10ml Kent Freshwater Plant 2x a week on tuesday and thursday.

Nitrates 20ppm
Iron (chelated) .75ppm
Phosphates 1ppm.

I would have thought my phosphates would be lower and that this is
probably
what is causing the BBA. Im going to stop dosing this until my phosphates
get a bit lower. This is my main priority; i want phosphates to be the
limiting nutrient in my tank. Currently it seems to be something else.

The PO4 is not what's causing your BBA. Stopping the addition of PO4 will
just make it worse. You have to figure out what nutrients your plants are
lacking. Because you're not adding KNO3 (and that's okay), you should add
more K2SO4 to make up for the K in KNO3. I'm not sure if you meant that you
were adding 20 ppm of K2SO4 or 20 ppm of K from K2SO4, but I would dose at
least 25 ppm of K per week just to be on the safe side. It will be dangerous
to have the PO4 be the limiting nutrient in your high-light tank. The
nutrient that you want to be limiting is light.

It is likely that the poor health of some of your plants and the appearance
of BBA are connected. Address the deficiency symptoms and the algae will
retreat. From your photos, it looks like it could be one of a number of
trace deficiencies. Are those symptoms occuring on new leaves or old leaves?
If new leaves are affected, it usually suggests a lack of traces. Old leaves
suggests a lack of macros. I would increase the Kent supplement a little bit
and see if that helps. But I would add some MgSO4 first.
__
Alex
pcalex (at) hotpop.com