Before you purchase your air pump make sure that you buy the right size pump for your pond. If you buy an undersized air pump you could hurt your pond more than if you had no aeration at all.
Aeration mistakes happen when you are not sure what size pump you really need for you pond. Under sizing the aerator can actually help grow algae and weeds. Algae only needs 2 mg per liter of dissolved oxygen to grow. Fish need double that. Even at 4 mg per liter fish are stressed worrying about breathing when they should be eating. They can survive but keeps them from growing bigger. The goal is at least 5 mg per liter of dissolved oxygen in the water from top to bottom of the pond. Getting oxygen to the bottom of the pond will help clean up the bottom muck. For additional muck reduction, add beneficial bacteria to control odors, murky water and excess nutrients.
The bad part of under sizing the aeration system is that soon you’ll see the algae coming back. Eventually it will return to the same situation and reason you bought the aerator. The aerator maybe doing its job but is undersized. Not being able to provide enough oxygen and enough water movement to do the job right, the nutrients will overtake the aeration system. Phosphates and Nitrates are not bound by oxygen content in your pond. So they can then become a food source for the algae and weeds once again.
Therefore a well aerated pond is always beneficial. However, in this instance, a little is not better than none at all so ensure that you buy the correct size air pump.