Choose big brands. The pre-filters of big brands like 3M in the United States are better and have higher standards. They are more reliable to use, but at the same time, the price will be relatively high. However, you can still get a lot of discounts if you choose the right place! For example, there is a group buying club in Beijing called Easy Packing. The 3M water purifiers there are several hundred cheaper than the price at the store, and you can also enjoy after-sales and services in the store. It is quite affordable. Water purification is only for use. Only then will you know how much it will improve the quality of life.