09-04-2012, 12:59 PM
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The problem with the Vallisneria is likely the water softness. And as Boredomb said, 100ppm equates to 5.5 dGH and that is soft water. I add Equilibrium to get my zero-hardness tap water up to 5 to 6 dGH. My Vallisneria does not do well. Moss loves this, I am frequently pulling out rafts of it.
As for plants doing better in one tank and not another, this is likely due to varying biological systems. No two aquaria will be exactly the same with respect to water chemistry/biology, no matter how much we treat them the same with light, fertilizers, etc. I see this too, with my 7 tanks. Even moving a plant like floating Water Sprite from one tank to another will cause it to decline for a few weeks due to something different. And my tanks are about as close to being equal as one could expect. But nature still rules the day.
I agree with the suggested plan of action, to increase Flourish Comprehensive to twice weekly (the full dose each time, which is 2.5ml (1/2 teaspoon) per 30g or 5ml (1 teaspoon) per 60g, whichever, and change the light. With T5 HO tubes, you are setting up a much higher balance with nutrients, and plants will always grow up to the limiting factor, which here was nutrients. Lessening the light intensity will result in a lower balance, that should occur with the increased Flourish.