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Young Jack Dempsey by Tony Terceira.
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Q. A few months ago, my pair of Jack Dempseys spawned. I successfully rescued the Jack Dempsey fry and have been rearing them in a 25-gallon tank. I’ve heard that Jack Dempsey fry generally grow very fast, but mine are still very small. Is there a way to make the Jack Dempsey fry grow faster?
Timothy Aldridge
Pasadena, California
A. The speed at which your Jack Dempsey fry (Nandopsis octofasciatum) grow depends on a number of factors, the most important of which are the amount of food they eat and can digest, the cleanliness of the water and the physical size of the aquarium in which they are housed.
Generally, it is believed that a faster growth rate is achieved by feeding frequently – at least several times a day with easily digestible foods – changing at least 50 percent of the water every day or every other day, and housing the fry in as large an aquarium as possible, keeping in mind the ability to maintain some density of food that allows the fry to feed without having to expend a lot of energy swimming around looking for it.
I’d suspect that feeding, water changes and physical space are all involved in the reason your Jack Dempsey fry are not growing very quickly. While you don’t say how many fry you’re trying to rear, I’d start by suggesting you put them in a larger aquarium – at least a breeder 40 (a 40-gallon aquarium measuring 36 inches long, 18 inches wide and 16 inches high). Then start with 50 to 75 percent water changes at least three times a week. Feed the fry an assortment of foods based on their size – chopped frozen bloodworms and chopped frozen brine shrimp are good foods for Jack Dempsey fry. I’d also add a prepared food, perhaps a flake food that you already use.
Be aware that you’ll have to increase both the size of the food items you offer as well as the volume of the water changes as the fry grow – you’ll also have to either divide them into smaller groups into a number of aquariums or periodically move the entire brood into a larger aquarium as they grow. An entire brood of Jack Dempseys will eventually take up a lot of space and time. Good luck!