Well, plant growth is contingent not upon light spectrum (though that does greatly effect our aesthetic appreciation of an aquarium) but rather upon the lumens it receives from the light source. For easy analysis, most people simply convert this to a measurement of watts/gallon of water. How many watts of lighting are you providing for each tank, and what size is each? I'm willing to bet that you have more watts/gallon in the 10 than in your larger enclosure. Elodea is an undemanding plant- it requires very little light, but will still respond with accelerated growth rates to higher light levels. The airstone has no factor in the growth, as plants only use CO<font size="-2">2</font> in photosynthesis, and, in truth, the turbulance produced by the bubbles probably degasses some CO<font size="-2">2</font> in the process, but that's not really a big deal for this plant.
Depending upon the fertilizer that you use, you won't need to rinse the plant before re-introducing it to your newt's enclosure. In truth, I dose fertilizers directly into my vivarium, with no adverse effects whatsoever (remember that most of these fertilizers were developed for use in planted aquaria, and if it's fish safe, it's almost guaranteedly safe for amphibians.) If you want to try some supplements, go with the Seachem line. Personally, I would advise Seachem Excel- it's a carbon supplement that will accelerate growth levels and stands in the place of supplementing CO<font size="-2">2</font>, so it'll save you some effort. I would also try to increase your light levels- you'll see an obvious jump in growth.
