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[APD] Nutrient problem in Echinodorus spp.

My 125-gallon tank has developed a nutrient problem that strangely only
seems to affect Echinodorus. My E. bleheri, E. parviflorus, and E. tenellus
all have a problem where the older leaves develop chlorosis and then start
to decay, finally leaving only a flimsy transparent gray skeleton of a leaf.

What is really strange is that this problem doesn't affect slower-growing
plants such as Anubias, Crypt. wendtii, or Bolbitis fern. Nor does it affect
faster-growing plants such as Hygrophila polysperma, Limnophila
sessiliflora, or Rotala rotundifolia.

The substrate is play sand over topsoil. There is no added CO2. Lighting is
100W NO fluorescent (4x F25T8 on electronic ballast). Temperature is 78F. pH
is 8.0 (measured right before lights-on, since it increases throughout the
day), GH is 12 degrees. PO4 is off the scale of my test kit, > 5ppm. The
only fertilizer I add is KNO3.

I first thought that NO3 shortage might be the problem, but I've been
running at 40 ppm NO3 for a month now with no improvement in the symptoms.

Any insights would be greatly appreciated!

Jim Seidman

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