I may as well tell you right away because there is no way you're going to guess.
No, not books. (Au contraire, mes amis.)

A new aquarium pump.

My (heated) aquarium holds a hundred litres and I've had some nasty algae problems. In particular the so called blue algae which is a combination of algae and bacteria, if I understand correctly, and very hard to get rid of except by using chemicals.
I recently did that. The tank doesn't look very happy yet. The blue crayfish in particular looks rather unhappy lately.


So I did some research, by reading up on the subject, and it seems that blue algae is usually a problem that rears its ugly head if the water is too still.
Hmm, I'm using a rather tiny filter driven by an air pump.
Furthermore, it seems that crayfish like streaming water.

Bingo!
*comic-book type lightbulb over head*
Appartenty I can improve both the water quality and the mood of my crayfish by getting a filter pump, one of the kind that pumps water, not air.
The one I just bought filters 700 litres in an hour; that means the entire water contents of the tank will be filtered SEVEN TIMES an hour. Sounds good, huh?

I think I'll install it tonight. And I'll be sure to tell you if the crayfish shows any signs of cheering up.