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Dying/brown leaves on Hygro

I have a strange (at least to me) nutrient deficiency (?) problem in my new 
75 gallon tank. It happens to fast-growing plants only; the most affected 
by far are H. Difformis and Anachris. The leaves turn brown and fall off, 
rather like they would if there wasn't enough light, but I know that isn't 
the case.

It takes a Hygro leaf about two days to go from healthy and green to brown 
and fallen off. Anachris is dying in a similar fashion, with the bottom 
half of the stems rotting away and sending the healthy growing tips floating.

The Hygro has what appears to be healthy new growth, but only crawling 
along the substrate. (This rules out low light as a problem.) Perhaps it's 
getting a nutrient from my substrate? That would seem to indicate Iron 
deficiency, but it doesn't look like the pictures I've seen.

This all started about three weeks after the tank was started. Before that 
the Hygro and Anachris (both transplanted from my other tank) were bubbling 
like crazy, but not growing very fast except for the low-lying Hygro leaves.

I see minor versions of the same symptoms on Giant Hygro and Limnophilia 
Indica. Rotala indica, on the other hand, is bright red and growing like 
crazy with no trouble.

My water has a high GH and KH. The pH in the tank is about 7.8. I am adding 
CO2 with a high-pressure system. I fertilize daily with TMG and a homebrew 
KNO3 solution. I have not had any positive results from Iron tests, but 
this isn't unusual. My Nitrate levels range from 2 to 5 ppm and are usually 
down to zero by the time I add the next dose of KNO3. The fish load is very 
low. Substrate is a mixture of Flourite and Onyx Gravel. I have Onyx Sand 
in a portion of the tank, and most of the healthy hygro is growing near the 

I assume this is a cycling-related problem, since my other tank has no such 
problems with the identical fertilizer regimen... Any ideas?

michael moncur   mgm at starlingtech_com   http://www.starlingtech.com/
"There it was, hidden in alphabetical order."           -- Rita Holt