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Growth problem with aquatic plants

Hi everybody. I' ve gotta question:

For some time now I have encountered problems with the growth 
of my aquatic plants. Although the plants do actually grow, the 
older leaves of the plants become curled and hard and 
sometimes drop off. Especially Lobelia Cardinalis shows these 
kind of symptoms but also Ludwigia Repens, Hygrophila Stricta
and Althernanthera Reineckii. The H. Stricta develops small 
brown spots on the older leaves which eventually drop off. 
I found in the archive of the aquatic plants mailing E-mail list 
a table by Neil Frank with nutrient deficiency symptoms and
 it looked as if there was a potassium deficiency. So I added
once a week a teaspoon of K2SO4 to my 500 l tank. I also 
used Dupla24 fertilizer (10 drops every other day) and 10 
drops every other day of my own PMDD based on EDDHA 
(sequestrene manufactured by CIBA GEIGY). CO2 is 
added to the tank by DIY sugar and yeastmethod. 

After adding the K2SO4 some plants really took off 
especially Hygrophila Difformis and Hottonia Inflata. But 
once again the older leaves became blotchy and ugly. 
Large holes appeared in the leaves of Barclaya Longifolia. 
Also beard algea and hair algea showed up. IMO it is clear 
that the plant are lacking one or more specific nutrients. 
Can somebody give me a hint of which nutrient(s) 
my plants are missing?

Here are some specifics of my 500 litre aquarium:
pH 6,8 - 7,2
KH 6 dH
Nitrate 12,5 mg/l
temp. 25 C
Fe level (no level detectable with my Dupla testset)

The substrate consist of gravel mixed with sand. 
Some loam is added. Also about 10 fertilizertablets are 
buried deep in the substrate. The tank is lighted by 2 x 
58 W Philips 830 and 2 x 58 W Philips 927 tubes 
(max 3 tubes or switched on at the same time). 
The ballasts are mounted underneath the tank to provide
some kind of substrate heating.

I am not certain if this is important but about a year ago 
I added some zinc-oxide (ZnO) to the water as a treatment 
for a blue-green algea outbreak (2 teaspoons). 
Could it be that the zinc-oxide has something to do with the
growth problems because it is still in the substrate?

I am looking forward to your respons.

Jan de Reus, Voorburg, The Netherlands