Learn Dutch: Het filter van het aquarium werkt helemaal niet. - The filter of the aquarium does not work at all.

Helemaal means 'completely', 'entirely', or 'wholly'. When you combine it with niet (not), it literally means 'completely not'. In English, the natural equivalent phrase is 'not at all'.

Why is the negation helemaal niet placed at the very end of the sentence?

In a basic Dutch main clause, when you want to negate the entire sentence or the main action (here, the verb werkt), the negation niet - along with modifiers like helemaal - usually goes at the end of the clause.

I have seen de filter before. Why does this sentence use het?

The word filter can actually be both a de-word and a het-word! When referring to a mechanical device or apparatus (like an aquarium pump or a coffee filter), het filter is the standard form in Dutch.

Since the current lesson is about using wat for 'a little', could I add it to this sentence?

You wouldn't use it here because helemaal niet (not at all) implies an absolute zero, which contradicts wat (a little). However, if you wanted to say it only works 'a little bit', you could drop the negation and say Het filter werkt nog wat (The filter still works a bit).