03-01-2012, 08:05 PM
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This may be a combination of several issues, so I'll mention each that occurs to me.
First is light. Stem plants are fast growing and thus require good light. [Pennywort is an exception.] I have a 29g tank too, with the single-tube T8 fixture by Aqueon (which may be what you have, doesn't matter the brand) which takes a 24-inch tube. The tube that came with it was junk and went straight to recycling. I use Life-Glo 2 tubes on all my single-tube tanks as it is the best I have so far found. ZooMed's Ultra Sun is also good, near identical. If you can find them in 24-inch length, the "daylight" tubes made by GE, Phillips or Sylvania also work, just get the one with a Kelvin of 6500K. These latter you can get in hardware type stores for a couple dollars; the first two I mentioned are in fish stores and more expensive. This will give you better light, but I can't guarantee Hornwort will do well, as we are still dealing with moderate light.
Second is nutrients; substrate tabs will do little for this plant, as stem plants assimilate nutrients via roots and leaves and roots will appear from most of the nodes along the stem. The node is the spot from where a leaf and/or root grows. So liquid fertilizer in the water is better. It needs to be a complete one to ensure all nutrients are available. I recommend Seachem's Flourish Comprehensive Supplement or Brightwell Aquatics' MultiFlorin.
Third point is placement. This plant, Ceratophyllum demersum, does best left floating [this solve the light issue]. This is likely why the lower portions fall apart when the upper get to the surface.
The higher temp might be an issue, but this plant should manage up to 30C/86F. It does not like chemical additives [Excel might kill this plant], but the two fertilizers I have mentioned are not chemical so they are fine. Last, many members find this plant falls apart on them, constantly clogging the filter with bits of leaf. Changing to another plant may be best, especially if you want the growth down in the water.
Hope this helps.