As previously mentioned, the key is balance. Light and nutrients have to balance. I do not recommend dosing the tank with ad hoc nutrients according to the Barr EI method because you don't have CO2 and light in balance for that, and the result will be even worse. Mr. Barr comments that his method is best in high-tech tanks with added CO2 and mega-light. In low-tech it is asking for trouble.
Nutrients are 17 in number, and include carbon (from CO2 mostly, sometimes from carbonates), nitrogen (mainly from ammonia/ammonium and less from nitrates) and various minerals in approximate proportion to each other. Plants grow when all these are present, but growth will cease if any one of these factors ceases to be available; this is referred to as Liebig's Law of Minimum which states that plant growth will be limited by the factor necessary for growth that is least available. When this situation arises, the balance is no longer present and algae sometimes takes advantage.
This is common in new tanks, within the first 3+ months, because the biological state of the aquarium has not yet settled. Once it does, this should not occur unless something happens to again upset the balance.
If your lights are T5 HO [High Output], you have way too much light and it will never balance the nutrients. CO2 comes from the fish and certain biological processes, and this is the one nutrient over which the aquarist [of a low-tech or "natural" setup] basically has no control [except by increasing/decreasing the fish load of course]. So the light and other nutrients over which you do have control have to be in balance. One watt of regular (T8/T12 type) fluorescent tubes per gallon is sufficient for good lush plant growth if the nutrients balance. You have a total of 30 watts over a 29g tank. But T5 HO tubes emit approximately 1.5 times the light intensity as regular tubes, so if you have T5 HO you are actually around 45 watts of intensity, and that is too much without everything else in balance, and I suspect your CO2 from the fish is nowhere near this level. So, reduce the light period, or consider different tubes.
If you doubt my advice on 1 watt being more than sufficient, check out the photos of my tanks [under my "Aquariums"]. I have exactly 80 watts of full spectrum (6700K) light over all three tanks. On the 115g and 90g this equates to less than 1 watt per gallon--and look at the growth of the swords. It's all about balance.
Last edited by Byron; 12-03-2009 at 12:55 PM..