Dick wrote in
:

What is "biological filtration"?

A natural process that occurs everywhere in nature that involves the
recycling of nitrogenous byproducts into various forms. The cycle is
incomplete in aquaria and generally is limited to the conversion of ammonia
into nitrite and nitrite into nitrate.

The first two byproducts are very toxic and can kill fish in short order
when found in quantity, the last byproduct is many times less toxic than
the others and generally only increases stress levels and the chance for
shock and infection when found in very high quantities.

In nature nitrate is further converted into nitrogen.

Without biological filtration every fish in your aquarium would be dead
right now.