Could or Can

This could be achieved using gravity to allow the water to flow from the higher to the lower level.

Sometimes students are taught in school that could is a more formal way to write can. It is not.

In English, can and could are used - most of the time - to show probability:

Can - Quite possible..
Could - Unlikely, but possible..

In English if something happens the same way, all the time, predictably, without variation, then it's 100% probable, and so can is more apporpriate than could. In this case, use present simple tense without can and without could:

This is achieved using gravity to allow the water to flow from the higher to the lower level.

Only use can or could when there is an element of uncertainty, and then choose either can or could depending on the degree of certainty:

In most situations this can be achieved using gravity to allow the water to flow from the higher to the lower level.

(Gravity certainly a good method)

This could be achieved using gravity to allow the water to flow from the higher to the lower level, but there are other, better methods.

(Gravity not necessarily the best method)

Notice that could implies a more negative evaluation than can. Again, Indonesians should think carefully about this distinction as they tend to over-use could, having been taught in school that could is more formal than can. Well, yes it is, but only in offers and requests:

Can you pass the salt? (informal)
Could you pass the salt, please? (formal)
Excuse me. Would you mind passing the salt? (very formal)