Myth #3 - Riparium plants require CO2 injection for vigorous growth.
The emersed aquatic plants in a riparium will be able to uptake more than enough carbon dioxide from the air, where it is normally many times more available than in water. There is no need to inject extra CO2 for riparium plants. I have seen a couple of riparium setups that used CO2 injection for underwater foreground plants. However, I have never tried this and I would be disinclined to do it. For the setups where I have used underwater plants I have included low tech selections (swords, crypts, Anubias
, Java fern) that grew very well without any extra carbon dioxide. By installing a CO2 system one would be losing out in part on the beneficial low input and cost-saving features of planted riparium.
Another tack that one could apply for the underwater area would be to use no plants there at all and instead develop a compelling underwater layout with a well-rendered hardscape, high-quality natural gravel and active fish display.
A related idea is the use of high-nutrient substrates for the underwater plants in a riparium, such as mineralized topsoil mixes with top dressings of other gravel substrates. I think that this would also be overkill in most cases. Topsoil plant substrates might be best used where underwater plant nutrient demands are very high, as with the use of bright lighting and CO2 injection. If planted into a riparium the underwater plants will usually be less demanding. The emersed riparium plants can throw a lot of shade on the underwater area and in a fully-planted riparium there will only be spaceand light for underwater plants up in the front 1/2 or 1/3 of tank depth. It is usually a better idea to select low tech underwater plants and feed them with some water column dosing or tablet or capsules fertilizers, rather than setting up a more complicated layered substrate.
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