How to prevent my cat from deconstructing cat fountain?

Try a different fountain.

You might like the look of that specific fountain, but it's made of lightweight plastic pieces that are assembled in a way that allows them to be easily moved by a pet (a lightweight plastic lid seated on top of the reservoir).

There are a number of different styles of fountain available, in plastic, metal, or ceramic. If you're determined to stick with a plastic fountain, look at how the pieces are assembled and try thinking like a cat--how can you push, pull, or knock this object apart? If you can't find a reasonable way, try that fountain, otherwise, keep looking around. If you decide to try a different material, you may have an easier task. Cats generally don't care to bite down on metal, and the ceramic ones are frequently too heavy for them to easily move around. They're also both easier to keep clean.

Once you find a fountain that they won't disassemble, consider how they're going to drink from it. Think about how your cats prefer to drink (do they prefer the bowl, a stream coming from the sink, or something else?) and how the fountain matches that desire of theirs. When you've found a match, make it available, but keep their still water available as well in case they still don't care for it. Some cats just don't like fountains, regardless of what cats generally seem to prefer.

Keep in mind that the aesthetics of the fountain should be the final consideration, below keeping it in one piece and make it appealing to the cats.