You have different varieties of fish that need different aquarium temperatures. Example: your zebra fish needs an ideal temperature of 64 to 74 F, whereas your betta needs 75-80 F. You need a different aquarium to provide such temperature. You also need to analyze your water. If the water is hard or with bad bacteria, then that is also going to impact their health.
Next Steps
Try to provide clean water with optimal temperature for each types