[dwalstad]

I tried pea gravel before and that was a nightmare! All of the roots rotted and plants floated to the top.QUOTE]

Be careful here to ascribe the problem just to a gravel layer. Uprooted plants mean that something is drastically wrong with your setup. It could be that the soil is too rich. The soil could be over-fertilized with sulfates or its too deep (greater than 1"). It could be that the plants were unsuitable or too weak or that the lighting was insufficient for plants to aerate their root area.

While I don't recommend this, I've used deep gravel caps and sand caps without problems.

Start with an unfertilized soil of 1" and cover it with as little sand or gravel that you can get away with. Plant good growers and give them plenty of light. Get some Malaysian Trumpet Snails, and/or poke the substrate with a sharp object to introduce oxygenated water.

If you've done all this and it hasn't worked, then something else may be causing the problem. In that case, more information and a picture might help.

 

[dwalstad]

If plants don't have enough light, will they begin the fermentation process?

Plants need oxygen produced by photosynthesis to keep their root zones aerated and detoxified. Fermentation is horribly inefficient, so the plants lose energy fast if they can't photosynthesize.

The problem you had with the 10 gal is a little less clear based on what you've written. Possibly the low lighting, pebble substrate, and no water circulation. I'm also wondering about your water's water hardness.

Glad to hear that your new tank is doing better. With that much light (15 watts CFL over a 5 gal tank) and a soil substrate, those Amazon Swordplants should have filled up the tank within a few months. Again, I'm wondering if your water is a little soft (lacks calcium, magnesium, etc). Amazon Swordplants really thrive in hardwater.