Magnesium

["Mark Trueman" <mark.trueman at agfa_com>]

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.

Tap water
Iron 0.017ppm
Nitrate 40ppm
Alkalinity as HCO3 289ppm
Calcium 134ppm
Magnesium 3.8ppm (*LOW??*)
Potassium 4ppm
Treating tapwater with Hagen Aqua Plus for chloramines.

PH with co2 injection in my tank is about 7.2

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.

Ive been dosing this for 4 weeks now and plants have been growing well but
bba has started to appear on some of my slow growers. I bought a phosphate
and iron test kit today and my tank is currently in the following state (its
a monday night)

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.

Ive also notices that some of my leaves are developing yellow edges and
brown veins which i initialy thought to be due to iron deficiency. Turns out
that this isnt the case (assuming a valid test kit) so i'm led to Magnesium
(or lack thereof) preventing the uptake of the iron.

QUESTIONS

Is it more than likely that magnesium is what im missing??

If so, what ppm of magnesium do i need to aim for?, How much epsom salts
would i need to add to my 160litre tank to hit this amount?

IMAGES
Some images may help

What is the problem here - http://www.marktrueman.com/images/brownvein.jpg
The veins of this plant are brown?? Any ideas???

http://www.marktrueman.com/images/irondef.jpg
Is the yellow leaf iron/magnesium deficiency? (Excuse the black molly in the
photo)

And heres a recent pic of the tank if you are interested
http://www.marktrueman.com/images/tank.jpg

Thanks for your help

Mark