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Aquatic Plant Aquarium Water Conditions

Aquatic plants are not very fussy about aquarium water conditions.

By Scott Hieber

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Aquarium Fish International Magazine
Q. I read your columns all the time but never seem to find much that can help me with specific water conditions. I’m looking for both freshwater plants and fish (and inverts) that will survive and thrive together in a 72-gallon, 42-inch bowfront aquarium under the following tap water conditions: pH: 8.0 to 8.4 (variable), hardness: 2 to 4 dH (variable), carbonate hardness: 12 to 14 dH (variable), no measurable iron (free or chelated) and no measurable phosphorus in any form. Lighting is 260 watts using power compacts.

The above water conditions were observed by frequent testing (at least weekly), over the last four months, in my home. I would appreciate any recommendations.
William Coxe
Baton Rouge, Louisiana

A. There is not a lot in my columns about water conditions because aquatic plants are not very fussy. They actually do well in hard or soft water, as a rule, provided that they have adequate aquarium lighting and nutrients. The water conditions you describe should be fine for plants, if you add nutrients.

First, let’s discuss the GH or general hardness, which is a measure of calcium and magnesium in the water. Your water appears to have a small but adequate amount. The alkalinity or carbonate hardness (KH) is moderately high, but that’s a good thing. When you add carbon dioxide to water, a small portion of the carbon dioxide forms carbonic acid, which tends to lower pH.

Adding some carbonates to the water has the opposite effect on pH, so those of us who have water with a low pH add carbonates to moderate the acidifying effect of adding carbon dioxide. As a bonus, a few aquatic plants such as Vallisneria species can get some of their carbon from carbonates in the water. Adding carbon dioxide is actually one of the best things you can do for aquatic plants; carbon dioxide is the primary source of carbon for them.

Water picks up carbon dioxide from the atmosphere and a bit from fish respiration, but those sources are unlikely to yield more than about 5 to 7 parts per million (ppm). If you add carbon dioxidecarbon dioxide, for example, by fermenting yeast in a bottle and directing the carbon dioxide that the yeast produces to the intake of your canister filter or the base of a powerhead submerged in the planted aquarium, you can raise the carbon dioxide level to 15, 20 or even 30 ppm. The aquatic plants will love it.

However, with the natural pH and KH of your water, you would not have to add any carbonates. You will end up with a pH in the mid 7.0s, which should be fine for most plants and freshwater fish.

The key nutrients besides carbon dioxide are nitrogen (N), phosphorus (P) and potassium (K). Given your water conditions, you should be adding phosphate. If the tap water doesn’t have significant amounts of nitrates or potassium, you need to dose those, as well. You can purchase commercially available nutrients that will supply the appropriate amounts of these elements (if you follow the directions on the container). You can also buy dry chemicals and add them with measuring spoons. Hydroponics supply houses are sources of these chemicals. Good choices for dry chemical nutrients are potassium nitrate (KNO3) for nitrate, monopotassium phosphate (KH2PO4) for phosphate and potassium sulfate (K2SO4) for potassium.

Aim for about 1 to 2 ppm phosphate, 10 to 20 ppm nitrate and 10 to 20 ppm potassium. These ranges and ratios do not have to be exact, but they are easy to remember and work well for almost all aquatic plants. You can use test kits to measure nitrates and phosphates. Otherwise, you can do measured doses by schedule and use large weekly water changes to ensure that nutrients do not accumulate too much. Dose twice a week, with the first dose right after a weekly change of one-half the aquarium water volume. Adjust the dose size for different sizes of aquariums.

For example, here’s the dosing, twice weekly, per 75 gallons: KNO3, one-half teaspoon; KH2PO4, one-sixteenth teaspoon; K2SO4, one-fourth to one-half teaspoon. This assumes that you have at least 2 watts per gallon of high-quality lighting, such as power compacts with good specular reflectors. With less light, cut the dosing to once per week. If you have 3 watts or more of lighting per gallon, dose three times per week.

The dosing does not have to be exact. Look at your aquatic plants. If they appear to need more, dose more often. However, if your plants are doing well and suddenly there’s a lot of green algae on the glass, cut back on the nitrate.

Also, use a good trace element mix. Trace mixes will ensure that all the nutrients plants need in tiny amounts, such as iron, are present. You can dose the trace mix according to the directions, but adding about 15 ml per 75 gallons two or three times a week ought to do it.

Without adequate nutrients, your plants will be weak, and algae will flourish. If you use the guidelines presented here, nearly any plant can flourish in your planted aquarium.

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