The differences between male and female goldfish will only become obvious once the fish reach maturity, which usually takes about a year.  However, the rate of maturation will depend on species and sex. Some breeds of male goldfish will reach maturity by the time they are 9 months old, while some females may take up to three years to reach maturity. Short of DNA analysis, identifying the sex of young goldfish is practically impossible. If you wish to increase your chances of having both male and female fish, your best bet is to purchase at least six healthy fish of the same variety. Statistically, there is a 98% chance that at least one fish will be a different sex from the others. Accurately identifying goldfish as either male or female is very difficult, and even the experts get it wrong from time to time. This is due to the fact that there are many exceptions to the rules:  Some male goldfish will not develop tubercles, while in rare cases a female goldfish will. Some female goldfish will not have protruding vent, while male goldfish sometimes will. In addition, some breeds of goldfish do not comply with general rules - for example,  some breeds (such as ranchu or ryukin) naturally have very round, thick body shapes, which makes gender identification through body shape almost impossible.     As a result, it is best to identify the sex of your goldfish by observing multiple signs, rather than trusting a single sign alone. Unhealthy goldfish may not follow regular spawning behavior or develop the usual physical characteristics that indicate gender. As a result, it is important to ensure the health of your goldfish (this includes things such as providing good quality water and fish food) before you attempt to identify the sex.  For example, an unhealthy male goldfish may not develop the tubercles usually seen during spawning season, while an unhealthy female may not have a convex vent. Body shape can also be misleading. A thin goldfish may be mistakenly identified as male (as males are usually smaller), but...

How would you rephrase that briefly using Vietnamese?
Biết rằng sự khác biệt giới tính sẽ chỉ xuất hiện khi cá vàng đạt đến giai đoạn trưởng thành. Biết rằng không có phương pháp chắc chắn nào để phân biệt giới tính cá vàng ngoài việc quan sát quá trình sinh sản của chúng. Hiểu rằng các phương pháp này chỉ đúng với những con cá vàng khỏe mạnh và được nuôi đầy đủ dinh dưỡng.