The most affordable freshwater live-bearer species of fish for home aquariums are typically guppies (Poecilia reticulata), platies (Xiphophorus maculatus), and mollies (Poecilia spp.). These fish are popular choices among beginner aquarists due to their hardiness, colorful appearance, and ease of breeding.

Here's a brief overview of each species:

1. Guppies (Poecilia reticulata): Guppies are perhaps the most common and affordable live-bearer fish available in the aquarium trade. They come in a wide variety of colors, patterns, and fin types, making them popular among hobbyists. Guppies are known for their active behavior and adaptability to a wide range of water conditions.

2. Platies (Xiphophorus maculatus): Platies are another popular and affordable live-bearer species. They are available in various colors and patterns, including solid, spotted, and variegated. Platies are peaceful fish that do well in community aquariums and are relatively easy to care for.

3. Mollies (Poecilia spp.): Mollies are closely related to guppies and are available in a variety of colors and patterns. They are known for their distinctive sail-like dorsal fins and peaceful temperament. Mollies can tolerate a wide range of water conditions, but they prefer slightly brackish water.

4. Goldfish：Without a doubt, goldfish are the most common ornamental fish that you may often see at your friends' homes. Goldfish come in various sizes and colors, allowing you to choose based on your preferences. Among the many varieties of goldfish, the common goldfish is the most prevalent, and you can find them at pet fish stores where they are likely the most affordable ornamental fish. They are incredibly hardy and adaptable, making them ideal for beginners!

If the temperature in your area is around 20 degrees Celsius, you will find that this fish is very suitable for you. You don't even need a heater. Just make sure to change 10% of the water every week.

These live-bearer species are generally inexpensive to purchase, and they often breed readily in home aquariums, providing hobbyists with the opportunity to observe the fascinating process of live-bearing reproduction. Additionally, because they breed easily, hobbyists can often obtain additional fish from their own breeding efforts, reducing the need to purchase new fish.

When selecting live-bearer fish for your aquarium, be sure to choose healthy specimens from reputable sources to minimize the risk of introducing diseases or genetic issues into your tank. Additionally, provide suitable tank conditions, including appropriate water parameters, filtration, and diet, to ensure the health and well-being of your fish.