The zebra danio is an easy-to-breed egglayer
Q. I have three zebra danios in my 20-gallon aquarium. One has a rather round belly. We assume that it must be a "she" and that "she" is pregnant. I tried to find some information online about this fish but have not found enough detail. The pet store told us to separate it from the others, and we have, but after some more reading, I'm thinking I should have the male and female together until the eggs are laid. This fish has been fat for some time, and someone told my husband they can be pregnant for a lengthy time. Is this true or not? Thanks!
-- Bernice Monk

A. You might be disappointed to find out that zebra danios can't become pregnant - they are not livebearers. But you'll be happy to hear that zebra danios are very easy to breed. They're probably breeding in your tank right now, although you won't know it because the tiny eggs and fry are eaten by the fish and sucked up by the filters. You also need to keep the males and females together to have success - although some hobbyists separate them for a time to condition them for breeding.

Zebra danios (Danio rerio, formerly Brachydanio rerio) scatter nonadhesive eggs amid plant leaves and substrate. The breeders quickly revert to eating the eggs, however, so a hobbyist who wants success breeding these fish needs to take precautions to protect the eggs. A popular method of breeding zebras is to take a bare aquarium of 5 or 10 gallons, and put a 2-inch-deep layer of marbles on the bottom. Fill with water to only 1 inch over the marbles. This gives the parents a shallow layer of water for swimming, and the eggs easily fall between the marbles, out of reach of hungry adults. After spawning, remove the adults.

But have you checked the price of marbles lately? Yikes! A 2-inch layer is far from cheap! Another method I actually like better is to hang a breeder net in a small empty aquarium (a breeder net is a net basket sold in aquarium stores). Get one with a coarse mesh so that the eggs can fall through - fine mesh will not work.
Fill the tank with fresh water (new water stimulates fish to breed) until only the bottom inch of the net breeder is underwater. Install a heater, and set the temperature to around 78 to 82 degrees Fahrenheit. You cannot use a regular filter system, because it will pick up the eggs and fry. An air stone attached to an air pump will work for now. Don't forget to dechlorinate your tap water with a proper water conditioner.

Now, you're set to introduce the breeding stock. Two males for each female are best. Males are slimmer than females, and their colors are often slightly deeper. Place the fish into the breeder net. When the fish spawn, the eggs will fall through the net and land on the tank bottom. (This is another advantage over marbles - you can actually see whether you've got eggs.)

Normally, the fish will spawn the following morning. Remove the adults and the breeder net afterward. The eggs hatch in one and a half to two days, but the fry won't become free-swimming for another couple of days after that (these times vary a bit with temperature).

Do not feed the fry until they're all up and swimming - you'll just pollute the tank. Zebra danio fry are very tiny and transparent. You'll have to look very hard to see them at first. This also presents a slight problem in that they are too small to accept baby brine shrimp as a first food. Look for the small containers of powdered egglayer foods sold at aquarium stores. Feed lightly several times per day, and change 10 percent of the water daily for maximum growth.

Nitrogen Cycle on Hold
Q. I have a question concerning aquariums in which the nitrogen cycle has already been established. I recently populated a new 20-gallon "long" tank with several gouramis in order to set the nitrogen cycle in motion. Several weeks later, after the ammonia dropped to undetectable levels, I removed the fish in anticipation of adding a large school of tetras. However, I was interrupted by other things, and the vacant tank continued to sit there with the filter and heater running. A month passed before I was ready to add my tetra shoal. Did I wait too long? How long does it take before the nitrogen cycle comes to a halt? How long can ammonia-converting bacteria survive without nourishment? Should I begin by adding only a few fish once again?
-- Jason Paloma

A. It depends. A couple of weeks isn't a big deal, but a month is long enough that you should play it safe and introduce fish in batches. If your tank is heavily planted, I'd go ahead and add all the fish. Live plants can use ammonia directly from the water. Indeed, heavily planted aquariums usually show no ammonia or nitrite spikes at all when first set up - even with a full load of fish. Having live plants also means it's likely that bits of vegetative debris and hitchhiking snails have continued to provide some food for the nitrifying bacteria while the aquarium was vacant of fish.

Gravel In Depth
Q. I have 5 pounds of gravel in my 10-gallon tank, and it doesn't look like enough. It varies in depth by about half an inch. Is that enough?
-- Tony Klynsma

A. Probably not. A typical rule is to use 1 1/2 to 2 pounds of gravel per gallon. This, of course, varies with the shape of the tank. For example, there are three common sizes of 20-gallon aquarium (20 extra high, 20 high and 20 long), all with varying sizes of footprints and needing different amounts of gravel to achieve the same depth.

Most hobbyists want to keep their gravel around 2 inches deep. It provides enough depth to allow plants to root, and it just looks proportionately right. For bigger tanks - say 55 gallons and larger, especially heavily planted ones - a 3- to 4-inch depth is even better and provides the same pleasing aesthetic appearance. If you use an undergravel filter, don't forget that the gravel is the filter media, and a 2-inch gravel depth is necessary for the filter to function properly.

Is deep gravel necessary? Not always. If you have no plants to root and don't keep fish that dig, covering the bottom of the tank is all that's really necessary, and your thin gravel layer would do. For example, a setup for blind cavefish might have stacks of rocks and no plants at all. Because blind cavefish don't move gravel around, you don't have to worry about areas of the aquarium bottom becoming exposed.

I recommend 15 pounds of gravel for a typical 10-gallon aquarium. I've included a chart that helps choose the right amount of gravel for other size aquariums.