04-26-2013, 10:18 AM
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I've never used these, but I would be careful using any outdoor pond products in an indoor aquarium. I've not looked into this myself, but it may be that the nutrients are not the same, or in the same proportions, or the products may break down differently (more on this below).
As for liquid and substrate fertilizers, both are generally needed, or if only one then the liquid is usually better. Aquatic plants take up some nutrients via their leaves, potassium, nitrogen, oxygen, carbon come to mind. A liquid fertilizer provides these into the water column, and as the water column does get into the substrate, the nutrients in a liquid thus cover all bases. So all plants, be they substrate-rooted or not, benefit.
Substrate fertilizers get taken up only via the roots in the substrate. Substrate tabs are not designed to break down into the water column, so they only provide certain nutrients to plants rooted in the substrate. This is where we come back to the pond products, which for all I know may break down into the water column. This could overload the water in an aquarium with nutrients, which could trigger algae or high nitrates.
Terrestrial fertilizers are not beneficial in an aquarium for some of the same reasons, but in this case they certainly contain different and unnecessary nutrients and different proportions of others.