Q. In addition to the two planted 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.
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 planted aquarium 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 planted aquarium water to feed the aquatic plants. Generally, even the well-rooted aquatic plants, like Amazon sword plants (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 planted aquariums 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 carbon dioxide, 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.