Personally, I have always associated the problem with a "dirty" tank. The kind that has high nitrates and low CO2. The kind of situation where the growth is stalled and it feels stagnant.

How's your CO2 and pH levels?

Light could play a role but most likely its low CO2 and lots of organics in the water. I wonder if there is a corolation between low O2 as a result of decreased growth, ie photosynthesis, and increased decomposition?

 

Well the tank is not full of organics. Its actually only 2 months old and I clean weekly. The Co2 is through the roof, but with aquasoil I am just making sure. KH=4 PH=6.0. I have been having troubles balancing this tank and have just recently got it to somewhat balance, so maybe I need to try again and wait.

 

You're using ADA Aquasoil?

What plants are disintegrating? Do you know your magnesium levels?

One thing I found that really stops stems from disintegrating at the substrate level is to put a Flourish Tab directly beneath the stems.

 

I had a similar problem and i found it to be because there was a fresh cut in the plant. I floated the plants for a week or so until some root formation was being produced then planted. The roots kep it healthy and growing as it should.

 

The most common reason for this to happen is damage. Stem plants are often damaged, cut, or crushed when planted into the gravel.

This can also happen when the base of the plant is dark, either from poor lighting or dense planting. I have seen it happen because of overly dense planting. I had a tank where the roots were starting die to occur on my Cabomba. I did a bit of gardening to space the plants apart and it stopped.

Interesting point Salt made about the root tabs in the substrate. A plant in a healthy, mature substrate may be able to support denser planting, further investigation is required. Intuitively, this makes sense to me. Plants can absorb nutrients from either the water or the soil. If the plant can't absorb nutrients directly from the water, it will try to absorb nutrients from the soil. If it can't do either, the root base dies and the plant separates. The plant may then move on to greener pastures. That's a theory anyway.

Update:
A second theory I've heard about dense planting is that it can block water circulation and cause dead spots in the water. The water parameters can be much different in these dead spots. For instance, I've noticed that the water is much colder, like 5-10 degrees, in the middle of a large mass of hair algae, where no water circulation occurs. It feels really gross when you stick your fingers into it.