Target phosphate levels

[maniki]

I'm currently cycling my tank, a 15 gallon with 36 watts of normal fluoro light.

It's quite well stocked with cabomba, myriophyllum tuberculatum and vallisneria (plus some little bits of h. difformis and ceratopteris).
http://i356.photobucket.com/albums/oo10/jdaus_photo/14112008.jpg

Water parameters are:
Ph: 8.1
Gh: 15
Amm: 0
NitrIte ~1 ppm
NitrAte 10 ppm
Phosphate: 0.5 ppm

(I get 10 ppm of nitrAte from the tapwater)

I noticed that growth had stopped a few days ago, that was when I first checked the phosphate levels, which were at zero. I've boosted them up to where they show up on the test kit (0.5), but should I be aiming for higher than that?

[Bk828]

Aim for 1-3

[Fordtrannyman]

Recommended Target levels have changed over the past few years.
I still stick with a Target range for phosphate of 1/2 to 1 mg/l in the aquarium.
My experience has shown that low diminishing FE levels stunt/slow growth.
In ever instance that I've witnessed this, N03 levels stayed about the same. This tells me that Iron is needed for the plants to uptake Nitrogen.

[maniki]

Thanks for the responses, I don't have co2 so i'll stick to the low end of the range, just under 1 ppm.

I was thinking it might be iron deficiency.. i've been dosing traces when I change the water, but as I can't find an iron test kit i'm a little wary about overdosing..

I think the plants must be getting their nitrogen mostly from ammonia rather than nitrate, as the nitrate levels have stayed constant..

[Fordtrannyman]

I remember reading some discussions on overdosing Iron earlier this year on another forum. I personally have never pushed the envelope to a maximum level. I tend to look for the minimums. At any rate I used a Seachem FE test kit to get a ball park idea.