09-04-2010, 05:19 PM
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When considering this question, one also must consider the amount required. Not only from the cost perspective, but adding "stuff" to a fish tank should preferably be minimal to achieve the goal.
One-half a teaspoon, or 2.5 ml, of Flourish Comprehensive treats 30g. It can be used once or twice a week (this is determined by the response of the plants in relation to the light, fish load, water parameters).
According to Tom Barr, the botanist who formulated the EI (Estimated Index) system of fertilizing high-tech tanks, Tropica worked well when he used 5ml for a 20g tank three times each week. That's roughly six times as much as Flourish. Now, I don't know how "planted" Tom's tank was, but he considered my tanks (from photos) were lush and thriving, and I have been using Flourish twice a week for almost two years.
The other issue that enters into any such comparison is the type of planted tank; low-tech requires less, high-tech considerably more in the way of additional nutrients.