There are several ways to tell if a betta fish is full:
Observe the feeding time: Generally speaking, it is normal for the betta fish to finish the food within 3 hours after feeding. If the food is eaten up in about 2 hours, it indicates that the feeding amount is insufficient, and you should appropriately increase the amount of fish food. If the food is still not finished after 4 hours, it means the feeding amount is too much, and you need to reduce it.
Check the belly condition of the betta fish: When a betta fish is full, its belly will bulge slightly, but not be overly swollen. If the betta fish's belly is abnormally bloated, it may have eaten too much, and you need to pay attention to adjusting the feeding amount.
Watch the betta fish's behavior: A full betta fish usually sinks to the bottom of the tank or hides in a corner to rest. On the other hand, a hungry betta fish will swim around frequently on the water surface or show signs of foraging.
Examine the defecation situation: After the last feeding, check if the betta fish defecates. If it doesn't, it may be "constipated", which is more common than ascites.
Pay attention to water quality and temperature: Maintaining suitable water temperature and quality is very important for the digestion and health of betta fish. Either too high or too low water temperature can affect the betta fish's appetite and digestive system. Generally, the water temperature should be kept between 24 - 28°C.