Cherry and Ghost Shrimp: These small, scavenging invertebrates are good tank mates for bettas in that they produce minimal waste, do not mind stagnant water, and can be kept in the same temperature range of bettas (though it will mean faster development and thus a shorter lifespan). Unfortunately, some bettas think ghost shrimp are a large, overpriced snack; if you see your betta harassing a shrimp, remove it promptly. You must also never use additives or medications containing copper to a betta tank containing shrimp, as it is highly toxic to these animals - as well as all other aquatic invertebrates.
Apple Snails: These colorful snails are an active, attractive addition to a betta tank. They are capable of surviving in lower oxygen environments, and tolerate tropical temperatures - with some reduction in overall lifespan due to the increased speed of development. They also adapt quickly to nipping and are generally too large for most bettas to kill or consume. However, the ideal conditions of both species is compromised to keep both in the same tank; apple snails do prefer oxygenated, filtered water and slightly lower temperatures. What's more, they demand for survival hard, slightly base water - the opposite of the betta's soft, acidic preferences.
African Dwarf Frogs: These tiny aquatic frogs are considered one of the best candidates for a betta's tank mate. The share the same preference for stagnant, warm, soft water, and are not as likely as other fish to attract a betta's aggression. However, it is essential to keep water conditions pristine, as they are very prone to bacterial infections with even moderate declines in water quality. What's more, as with invertebrates, African Dwarf Frogs have no tolerance for copper. Finally, be sure your ADF is getting sufficient nutrition; because both bettas and ADFs are carnivores, they will be at odds with each other for food competition. It may be wise to feed the betta or frog in another container, as food competition can illicit aggressive behavior.
Otoclinus Catfish: Many betta keepers house otoclinus catfish with their bettas. These small, low-waste fish are fantastic additions to planted tanks and greatly reduce algae. They are fast without being distracting, dully colored, and do not share the same water space as bettas - thus, they seldom encourage aggressive responses from bettas. However, they require an extremely stable environment, and thus require a matured, cycled tank - preferably planted. When stressed by fluctuations in water temperature and parameters, their digestive systems may shut down. It is also important that harassment from the betta be monitored, as this form of stress can be equally serious. Finally, remember that otos are a social species that prefer living in larger groups.
White Cloud Mountain Minnows For a tank mate more dynamic than bottom feeders, the white cloud mountain minnow is often an acceptable choice. This small, peaceful fish does not tend to nip fins, and can usually escape a betta's aggression with ease. They are very hardy, and inexpensive to purchase. However, they also prefer cooler water, so your betta's tank will need to be kept at the lower end of the preferred range. Also, a tank of at least 10g is preferred to 5g in this instance, as WCMMs are very active; your betta will need plenty of hides and plants to avoid feeling stressed.
MAYBE"S
Neon Tetras: In many ways compatible with bettas, neon tetras share a similar preference for pH, temperature, planting and light conditions. However, they may nip fins, and a minimum of six fish is essential - meaning no tanks under 10g
But i suggest that if you do decide to add any that you moniter very carefuly for the first couple of days to weeks.
Thanks RandomWiktor from Ultimatebetta.com