When purchasing fish, deciding whether to buy adults or juveniles might seem simple at first thought. However, you should probably think about it a little bit. Why? Because the answer should really depend on why you want the species and what its value is to you.
Unless you’re infinitely familiar with the species you’re buying, how do you even know if it’s an adult or a juvenile? For example, Neolamprologus brevis are very small cichlids, or dwarf species as we call them. A full grown adult brevis at ~2.5 inches is smaller than juveniles of many other species.
You should also consider the species lifespan. Some species live much longer than others. Furthermore, many species aren’t sexually dimorphic until they’re adults. In other words, you might not know for a while what gender you have.
Using the brevis example, if you go online and purchase a full grown adult, you really have no idea whether that fish is a year old or three years old (unless you’re an expert on brevis). So in terms of value, unless you can determine the age of that full grown adult, how do you know whether you’re buying a fish that, assuming it’s healthy and will live its normal life span, will live for four months or three years? I don’t know about you, but I don’t want to pay $15-20 for a fish that might well die of old age in four months versus paying the same price for one that is only a year old and has several more years to live. Thus, if I can’t really determine the age of the fish once it’s an adult, I’m better off buying what I know is a juvenile. So back to the brevis example. If I don’t know much about that species, but I know for, say $10, I can get either a 1/2″ fish (which would be a juvenile) or a 2″ fish (which would be an adult), what makes the most economic sense?
There are certainly exceptions to this. For example, if you’re wanting to breed a species immediately and it’s a species you don’t currently have, you’ll want to purchase adult fish. But even then, you have to be careful. A female cichlid’s fertility and even fecundity typically decrease as she ages. So knowing exactly how old the fish is might still matter even if you know it’s already reached breeding maturity.
These are just a few of the factors that you might consider when you decide to buy a fish.