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Converting a Fish-Only Aquarium to a Planted Aquarium

There are only a couple of things you need to do to convert your fish-only aquarium to a planted tank.

By Scott Hieber

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Q. I want to change my aquarium from a fish-only setup to a planted aquarium. Do I need to take any special steps so I can grow plants?
Jasper Kolbe
Dassel, MN

A. There isn’t a lot that one has to do to be able to grow aquatic plants in an aquarium. The key things that will be different from a fish-only aquarium are lighting and nutrients. You will probably need to add more light but not necessarily more nutrients. Optionally, you can add carbon dioxide.

A first concern is lighting. Usually, fish-only aquariums are set up with enough lighting for viewing: roughly about a watt or so of fluorescent lighting per gallon. Aquatic plants need more light — about twice that amount — with high-quality specular reflectors. With plain white plastic reflectors, 2½ to 3 watts per gallon works better.

The next concern is nutrients. When keeping only fish in an aquarium, we usually try to maintain the nitrates and phosphates at a minimum to avoid algae growth. Some folks even put phosphate removers in their aquarium filters. In a well-planted aquarium, you do not want to remove the nitrates and phosphates; the aquatic plants need these nutrients and will absorb them. You can easily maintain levels of nitrates and phosphates at 10 ppm and 1 ppm, respectively, without any fear that this will induce any algae blooms — if the aquarium has a lot of aquatic plants. If you place only one or two plants, or a few strands of Elodea in the aquarium, you will probably end up losing in both directions, having enough nitrates and phosphate to feed algae but not enough to feed the plants. So, using a lot of aquatic plants is key.

Fish food can be an adequate source of plant nutrients if there is minimal lighting, but if you have 2 or more watts per gallon or you add carbon dioxide, the plants will start to grow faster and use up nutrients quicker. In that case, you will want to supplement by adding nitrates and phosphate. If you keep the aquarium clean of algae when it appears, you can maintain relatively high levels of nitrates and phosphates, and have very little algae to deal with — much less than in a fish-only aquarium.

Some aquarists figure that more light is better, which is true only up to a point. Once you add more than about 2 watts per gallon, algae can be more difficult to control, unless you add CO2. Carbon dioxide is an essential nutrient for plants. Terrestrial plants get CO2 from the air; aquatic plants have to settle for the amount of carbon dioxide dissolved in the water. Some CO2 naturally dissolves from the air into the water, and that is adequate, unless you try to use more than 2 watts of light per gallon (if you have stronger lighting, the aquatic plants will photosynthesize more, depleting more nutrients, including the small amount of carbon dioxide that is available from the atmosphere).

Carbon dioxide can be added using commercially available yeast fermentation systems or by using compressed carbon dioxide.  However, added CO2 is not essential if you maintain lighting at my recommendation.

On the other hand, aquatic plants will benefit from added carbon dioxide even if your aquarium does not have an intense level of lighting. Also, added CO2 does not spur algae growth.

You do not need any special substrate, as long as you have sand or gravel that can hold rooted plants. Aquatic plants can get nutrients directly from the water column; however, aquatic plants can feed from their roots, so plants with roots will take advantage of nutrients in the substrate.

Regular large water changes are a good idea for fish-only and planted aquariums to remove the organic compounds that can build up over time. So, there’s no reason to avoid adding plants to your aquarium.

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