08-04-2009, 07:48 PM
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This is the opposite of what I would expect--it should be better in the 55g which has more light and nutrients. Rhonda Wilson, who authors the monthly "The Planted Tank" column in TFH, says ludwigia is easy to grow but requires good light (2 watts per gallon minimum) and nutrient fertilizer. It will grow in soft or slightly hard water, but prefers soft and slightly acidic. All of this is what you describe for the 55g, but that is the tank in which it is not doing so well.
The type of tubes over these tanks may have a bearing on this; not all "plant' light is good light for plants, unfortunately, and plant light tubes tend in my experience to be less intense so it penetrates deeper tanks less adequately. I'm wondering if the single tube on the 20g may perhaps be a better one (or newer?), and in the shallower tank the light is more intense. Light is the single most important factor in aquatic plant growth. Fluorescent tubes need to be replaced at least once every 12 months, and some sooner. The intensity of light output decreases quite rapidly during the first few months of use. I recall Karen Randall once suggesting that every 6 months would probably be better, but of course that gets very expensive, so she advocated replacing the two tubes alternately to partially offset the decrease in intensity from both tubes during the last half of their time.
Another suggestion would be to check the spectrum graph for the tubes you have; the Drs. Foster & Smith website has the spectrum graphs for many (perhaps all) of the tubes they list, yours may be there, or try the manufacturers' websites. Plants require blue mostly, then red (which is why the "plant' tubes give that purplish hue); green is useless to plants but usually added to make the colours appear natural to us.
One last suggestion would be to replace the plant lights on the 55g with full spectrum. The Hagen "-Glo" series is good, and there is the less expensive Zoo Med series, and the Lightning Rod series. Choose full spectrum like the Life-Glo or Ultra Sun, the closest to mid-day sun (which is 6500K).