Air stones come in all sorts of shapes and sizes, the most common ones being the flat ceramic and the tubular ceramic,
The better quality ones always last much longer and are heavier in weight so they don't flat about on the bottom of your pond, some of the cheaper Korean stones will lift and flap about, many koi keepers these days are using the long rubber air diffusers because they give you a finer bubble and gives you more dissolved oxygen and a nice curtain of air, Make sure not to overload the rubber diffusers with too much air as you will wear them out quickly,

Always make sure that the type of stone or diffuser that you use has no rough edges and that metal clips soy ou koi will not be able to damage themselves, always use a good quality air pump something that will last you years and that you can obtain replacement diaphragms and air filters,

Aeration" is the term that we use to mean adding air to the water.
Since air contains 22% oxygen, aeration adds oxygen to the water. There are several very
essential pieces of equipment to consider when planning a pond. None more so than a good reliable quality air pump.

So why aerate a Koi pond?
A Koi pond is more than a hole in the ground with a liner and some water. To have a healthy, dynamic pond environment you also need a biological filter, a source of aeration, a circulation pump and adequate space, or volume. It is important to note that all of these elements must work in harmony with one another.