02-25-2013, 02:06 PM
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A soil substrate in theory should provide sufficient nutrients without substrate fertilizers. And before considering substrate additives, I would look at a liquid supplement. Some nutrients are taken up via the leaves, not the roots, and these must be in the water column.
I have maintained swords well in tanks with plain gravel or sand with just liquid fertilization. For the larger species, I add a substrate tab next to it, in addition to the liquid. While this does certainly benefit with faster growth, it is not necessary for a healthy plant. But your soil to deal with the root nutrients anyway.
A complete liquid like Seachem's Flourish Comprehensive Supplement works well; once or twice weekly. Start with once, and after a few weeks depending upon the plants' response, you can dose it twice if needed.
The lighting is important, obviously; heavy feeding plants need sufficient light. You mention two 15w hoods, but what exactly is this? Fluorescent tubes at 15w each, or screw-in bulbs at 15w each? And what is the Kelvin of the tubes/bulbs? And what sized tank, to put this in perspective?