11-05-2012, 01:21 PM
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I would go with just the liquid for a few weeks and monitor the plant's response. Liquid is essential if any fertilizing is necessary, for any plants; some of the nutrients are only taken up via the leaves, so not having them in the water column is not going to help much. And the water naturally gets down through the substrate, carrying nutrients with it, so all plants will benefit from liquid.
As Boredomb mentioned, some plants may need additional substrate fertilizers. These are plants we term heavy feeding, such as the Echinodorus (sword) species. I use liquid (Flourish Comprehensive) in all my tanks, plus in the two with my large Echinodorus bleherae, E. cordifolius and the Tiger Lotus, I use a tab next to each plant, once every 3 months. The species mentioned do show increased growth from the tabs, compared to when they only got the liquid.
Vallisneria should be fine in Eco-complete with regular liquid fertilization. It's primary need is for "hard" minerals, as it uses bicarbonates a lot and thus thrives in harder water.