Unfortunately, you are not recommended to do so. As different fish requires different water hardness and pH level to survive, it is not advisable to keep them in the same tank. For example, a Lake Malawi Cichlid requires hard water and pH values of 7.7 to 8.6, while an Amazon River fish requires softer water and pH level of 3.8 to 4.9. If your aquarium’s pH is too high or low it may be very stressful for your fishes. Hence, it is important to know what kind of environment is suitable for your fish.