My new female platy looks pregnant. What to do about male aggression?

She may have had too many babies developing and sacrificed some eggs in order to have the rest of them develop properly. I would not upset her now by removing the eggs. I suggest you leave her in the breeder box until she has the rest of the babies. Ignore the eggs on the bottom of the box, It is likely that they will not survive. But Platys have young all the time, so it is not such a problem. She will have live young soon.

Unfortunately, aggression between species of fish is not uncommon. I am not sure if this is a new finding, or if anything has changed in the dynamic in your school of fish (any new introductions or any deaths of fish), but this often can then change how the rest of the fish interact with one another. You can try to give the platy places to hide and escape from the guppy, or you can end up dividing the tank. If you do not want to add dividers, you can also separate the guppy into another tank as well. I hope that gives you a place to start, and good luck!

She looks like she may be pregnant. A female platy will become quite large when pregnant. Gestation lasts about 28 days. When she's ready, she will swim to a quiet part of the tank and give birth. The fry are fully developed and able to swim. If not removed soon, they may get eaten by the mother or other fish in the tank.

Both species are excellent "beginner" fish and quite easy to care for. Your 20 gallon tank is a good size for about 6 to 8 fish of this species. 10 would be close to overcrowding the tank. Once your tank is properly cycled, you can add your fish. They are omnivorous and will readily eat flake, pellets, freeze dried and live foods. Chemistry should be monitored regularly and kept as follows: ammonia 0ppm, nitrites 0ppm, nitrates 20ppm or less, pH about 7, water temperature between 76 F and 78 F. Provide moderate cover (hiding places). These fish are live bearers, so if you have male and female then expect fry (baby fish) at some point. If not separated, fry will likely be eaten by adults.

If the tank was not properly cycled before she went in, the water qualityu may be affecting her. Check the water parameters first. Temperature should range bewteen 68 F and 78.8 F (20C to 26C). Using a home test kit, ammonia and nitrites should read 0 ppm. Nitrates should be 40 ppm or less. Platys do best in a pH of 7 to 8.3. Adjust parameters as needed and see if it makes a difference in her behavior. Offer a variety of appropriate pelleted and flaked food as well as freeze dried to see if she has a preference. Platys are livebearers with a gestation of around 28 days. She will become noticeably bigger as her date approaches. If she is pregenant and has fry, you will need to separate her or give the fry plenty of cover in the tank. If not, she will eat them. The stress of a new tank and water may also be a factor. Hopefully, with some adjustments and different foods, she will perk up.

On average, a platy female can spawn 20-50 fry (baby fish). The first spawning may be less. If she still appears bloated or round, she may still have fry to spawn. If no more appear after a day or two and she is back to "normal" size, then she can be returned to the main tank. The fry should remain in a grow out tank or they will likely be eaten.

Beta fish lay eggs and she would need a male to then fertilize those eggs after she lays them. Breeding betas can be very work intensive. Here is a link with great information: fishkeepingworld.com/breeding-betta-fish/ As the babies get bigger, they can graduate to larger food. The babies will mature and need to be separated around 3 months of age. The females can stay together but all the males will need to be separated. Breeding betas requires a lot of tanks/space. I recommend to connect with a local aquarium store and find out if anyone has first hand knowledge. I hope this helps and please feel free to post any additional questions.