I have not used the API tabs myself, but I have heard unfavourable comments from those who have. If these are disturbed, as during water changes or by digging, apparently they can make quite a mess. As for their effectiveness...without knowing what they contain it is difficult to know how effective they may be, and there is not much info on their website. It does mention iron, potassium and carbon and the carbon gives me cause for concern. I know their liquid product, Leaf Zone, only contains iron and potassium and while that may help it will only do so if the other 15 required nutrients are present in sufficient amounts to provide for the plants' requirements.
I have been using the Flourish Tabs for five years now, with good results. I only use them next to certain plants, not throughout the substrate, which is not necessary. Organics (fish excrement, etc) enter the substrate and break down, providing the major source of carbon (CO2) and other nutrients for plants. Adding a complete liquid fertilizer is all that is necessary beyond this, in low-tech or natural method planted tanks. Plants can only take up nutrients from the water, whether via roots or leaves, so adding the fertilizers to the water will cover everything, and the water does circulate throughout the substrate. The benefit of substrate fertilizers is that they take longer to enter the upper water column (Seachem says this does not even occur with their tabs) which is less likely to create algae issues, and if the product is a good one the essential nutrients are being released slowly and in close proximity to the roots of plants that need them.
I use one next to each of the large sword plants in three tanks, and one next to my Red Tiger Lotus bulb and one per Aponogeton, and the tabs have made a noticeable difference to these plants. But I do not use them elsewhere, and the crypts and chain swords are thriving.
I mentioned algae earlier. The trick to controlling algae in a planted tank is to have the nutrients in balance with the light intensity (and duration), suitable for the specific plant species and numbers. Some plants are especially heavy feeders--the Echinodorus species (swords) for example--and this is where the substrate tabs are so helpful. These add additional nutrients beyond the liquid fertilizer, but not directly into the water column. Adding too much liquid fertilizer can cause serious algae issues because it takes longer for this to be taken up by these larger plants rooted in the substrate. In a tank with more faster-growing plants such as the stem plants and floating plants, which rely much less (if at all) on what is in the substrate, increasing the liquid fertilizers may be effective. But more than once I have increased this beyond the needs of the plants, and algae has always increased. I've done sufficient experiments with this to be certain.
Byron.
