Danger of keeping large amount of fish together

It's never usually a good practise to have a large number of fish stocked up in a single tank. The reason for that is if any illness strikes, it strikes with a bang and can be more fatal than otherwise. Heavily stocked tanks are a perfect place for any disease to spread and thrive and persist which makes the disease more fatal than it actually is. Some examples are diseases like ick.

In situations like this, you should immediately reduce water level and start medicating with the right medicine for the problem with the correct dosage. Beware of overdose as that can easily kill your fish. Read the dosage and apply the right amount depending the volume of water in tank.

Do regular water changes. Make sure any dead fish are removed immediately.

The Convict Cichlid (Kindle Edition Ebook)