Q. I have a 90-gallon planted aquarium. After about a month, my plants seem to develop brown spots. My Anubias seems to be the healthiest. I have noticed the problem especially with Amazon and ocelot swords. Here are my conditions: 110-watt compact fluorescent half-blue actinic daylight, nitrates usually around 0-20 ppm, nitrites 0-5 ppm, pH 6.8-7.0, ammonia 0-.25 ppm, hardness 100 (KH). I also have a CO2 dispenser. My substrate is 50/50 Flourite/rocks, and I do a water change 25 percent every two weeks. Do I have enough light? I am at a loss as to why my plants aren't doing well.
-- Scott Clifton

A. I think your swords are hungry for food and light. Plants from the fish store, especially when fresh from the aquatic nursery, are stocked with nutrients, and they can survive on these for several weeks before they use up these stores and become deficient in nutrients. Fast-growing plants like most commonly available swords, will use up their stores quickest, while ponderously slow-growing plants like Anubias will do so very quickly. In fact, because they grow so slowly, their nutritional needs are much lower than swords.

Rule-of-thumb targets (in parts per million) for potassium, nitrates, and phosphates are, respectively, 20, 10 and 1. In an aquatic garden with less than 2 watts per gallon of lighting and no added CO2, you could cut those levels in half, and most plants will do fine. If you do not want to test for nutrient levels, you can dose with commercially available products and do large weekly water changes to prevent accumulation of excess amounts.

About the Author Scott Hieber has kept aquaria since he was 11 years old, "back in the metal frame days." He turned solely to planted tanks about five years ago, maintaining about a dozen at his home and workplace. In his enjoyment of living aquaria, he balances his penchant for simple, easy-to-maintain setups with a lifelong interest in things electric and mechanical. He serves on the Board of the Aquatic Gardeners Association. He has traveled to the Amazon to see tropical fish and aquatic plants in their natural environs. Originally from southern California, Scott now resides near the New Jersey central coast. He says he went east "for the weather."

If you have a lot of plants garden, not just a few plants here and there, then ammonia and nitrites should be virtually unreadable with most high-quality test kits. With low-quality test kits, the readings are not accurate enough to be useful in these ranges. But if your plants are hungry and their growth is stalled, these chemicals can reach higher levels.

I would prefer about twice the amount of light that you have and to run those for about 10 hours per day. Since you are adding CO2, you should have no problems going a little bit higher than 2 watts per gallon, but try for something at least close to 2. The Anubias plants, as you have found, are quite content at low light levels; they grow very slowly and don't have much use for high levels of light. But swords are naturally fast growing plants that need more light to flourish than Anubias.

Also, unless you are fond of the coloring the actinic gives your garden, replace it with a broad-spectrum bulb intended for planted aquaria. Although plants can use a wide range of wavelengths, including much of what is emitted from actinic bulbs, these bulbs tend to put out less useful light for plants. They are designed to put out a relatively high amount of the very high energy wavelengths that reach down through several feet of ocean water to coral reefs. However, making a bulb that emits a relatively high amount of the far blue end of the spectrum usually means the bulb produces relatively less light overall that the plants can use.

Rabbit Pellets for Plants
Q. In addition to the two aquariums I currently own, I keep a few rabbits. I have heard that burying a few rabbit pellets in the aquarium substrate near plants is a good way to fertilize the plants. Is this safe? The rabbits are in good health and are fed organic food. Would I need to use other plant supplements in addition to the pellets?
Thank you in advance.
-- Daniella Lowery

A. Depending on what you mean by "pellets," it could work. If you mean high-quality pelletized, commercially prepared rabbit feed, it will be very high in fiber and not much protein. They are clean and a mild source of nitrogen. So, the risks of releasing too much nitrate into the water column are small. However, they have to rot in the substrate before they become useful to the aquatic plants, so benefits will be less than with fish food, possibly supplemented with aquarium plant fertilizers. Lower-quality rabbit feed contains alfalfa, which is lower in fiber, and presents greater risks.

If you mean the rabbit excreta, this is not advisable. By nature, rabbits are foraging herbivores. They take in a high volume of food, and it has a fast transit time (about 4 hours). So, fecal pellets are high in harmless fiber, and the other constituents are not highly processed; however, the fecal matter is high in various bacteria. Adding bacteria from mammalian feces to a tank of standing warmed water is not a healthy idea.

In either case, you would be risking an algae bloom if the substrate is disturbed by gravel vacuuming, or planting and transplanting. Some aquatic gardeners successfully use Jobe's plant stakes in the substrate, but concentrations of nitrates in the substrate can cause an algae bloom if the substrate around them is disturbed. You would be better off using either a commercial aquarium plant fertilizer that can be added as a liquid, or dry chemicals such as potassium nitrate, which can be inexpensively purchased from hydroponics supply vendors. Either of these can be added directly to the aquarium water to feed the plants. Generally, even the well-rooted aquatic plants, like swords (Echinodorus), do perfectly well taking up nutrients from the water, so there is little need to put concentrations of nitrates in the substrate.

As for other nutritional needs, it depends in part on what is already in your tap water. You can find this out by asking your local water supplier. If it has more than 0.5 parts per million (ppm) of phosphates, don't add any more. If it has 5-10 ppm of nitrates, don't add any more nitrates. At a minimum, I would add a good trace mix, such as Tropica Master Grow, SeaChem Flourish or a similar product, three times per week (at least) at the stated dosage. If you have less than 2 watts per gallon of fluorescent lighting and are not adding carbon dioxide (CO2), fish food might be the only source of nutrients that you need in addition to the trace mix. In tanks like these, all I add is trace mix three times per week and a little potassium with each water change.

However, if you have stronger lighting, add CO2, or have very few fish in the aquarium, then you might need to add some potassium, nitrate, phosphorus, or all three. If your plants seem not to flourish, and you have repeated and seemingly obstinate problems with algae growth, you probably need to add one or more of the three nutrients that plants need most: potassium, nitrogen, and phosphates. The easiest way is with commercially available liquid forms of these nutrients.

Each aquatic garden will differ somewhat from the next in the amount of  nutrients it needs, and you learn by seeing how your plants are growing. There are two important things to remember about fertilizing aquarium plants. First, maintain a balance between the nutrients (see the response to previous question for more on dosing). Too little phosphate, nitrates, or potassium will weaken the plants, stall growth and give one or more kinds of algae a chance to flourish. If you are concerned about dosing too much, change 50 percent of the water each week, and dose with the water changes. The large water change will keep the nutrient levels from accumulating to undesirable levels. Also, they will keep organic compounds from accumulating.