Learn Dutch: Ik ga wat voor het aquarium kopen. - I am going to buy something for the aquarium.

While wat is indeed the question word for "what", in a normal statement like this one, it is very commonly used as a casual, unstressed version of iets (something). So Ik ga wat kopen simply means "I am going to buy something".

Could I say Ik ga iets voor het aquarium kopen instead?

Yes, absolutely! Using iets is perfectly correct and means the exact same thing. wat just sounds a bit more conversational, almost like saying you are going to buy "a little something".

Why does wat come before voor het aquarium? Could I put it after?

Putting wat first is much more natural. Because wat is a short, unstressed pronoun, it prefers to sit early in the sentence, right after the conjugated verb (ga). The longer prepositional phrase (voor het aquarium) is placed after it.

Why is kopen separated from ga and placed at the very end of the sentence?

This is a standard Dutch word order refresher. When a sentence has two verbs, the helper verb (gaan) takes the second position (ga), and the main action verb in its full dictionary form (kopen) gets pushed to the very end of the clause.