There are few things more discouraging to a breeder than finally getting a favorite pair of fish to spawn, only to have all the fry die days or weeks later. Unfortunately, it's not an uncommon scenario-but it is one that an aquarist can often avoid if he or she has a good understanding of the special needs that fry have and is con­scientious about meeting them.

 Water Quality

Fry are far more sensitive to water quality than most adult fish, and even small amounts of ammonia or nitrites, or excessive levels of nitrates, can kill them or stunt their growth. So it's extremely important to make sure that the water quality in a rearing tank is as close to perfect as you can get it.

 That means making sure you don't overstock or overfeed, and doing regular water changes-in other words, all the same things you do to maintain adult fish. It's just that with fry, it's a little more important to get the balance right.

 That's because fry need lots of food to fuel their development. And lots of food equals lots of fish poop and other decomposing organic material in the water, which, untended, equal ammonia, nitrites, and nitrates in quantity.

 The best way to prevent this, of course, is to do small but regular water changes, making sure to siphon out all the detritus from the bottom of the tank in the process. (That is one of the reasons rearing tanks rarely have substrate.) In a fry tank, it's not uncommon to change 10 to 25 percent of the water every day or every couple of days-being careful, of course, to match the pH, hardness, and temperature of the new water to that of the old, since fry are highly sensitive to changes in their environment.

 Be careful, too, not to vacuum up fry, which can easily get caught in the pull generated by even a very small gravel vacuum. To protect them, take the "bell" off the end of the vacuum and just use the tubing, or for very small fry, use airline tubing. You can create an additional safety net for the fry-both literally and figuratively-by fitting a clean piece of nylon stocking or gauze over the end of the hose and fastening it with a rubber band.

 A Word On Stocking Density

Studies have shown that fry kept in overstocked tanks are at risk of stunted growth and may never reach a normal adult size. In worst-case scenarios, overstocking can lead to outbreaks of bacterial disease, and even deaths of some or all of the fry.

The problem is that the definition of "overstocked" changes as the fry grow: A 10-gallon tank may easily support hundreds of fry in their earliest stage of development, when they look like nothing more than snippets of thread with eyes. But as they get bigger, they will put an increasingly heavy load on the filter, which will in turn affect water quality. So be prepared to move some of them to other tanks before this happens, and maybe even to repeat the process a couple of times before they're grown.