They can be placed above the water level

The pump is hooked onto the outside of the aquarium, but it can also be submerged. External pumps tend to be more powerful and will not heat up the surrounding water, while submerged aquarium pumps must be monitored for heat generation. The size and power of both types of pump are directly proportional to the size of the aquarium, as well as the amount of water that it contains: A larger tank requires a more powerful pump to circulate water at the same rate as a smaller aquarium equipped with a less powerful pump.

Here are the list for Top 5 Best water pump aquarium small that you can purchase on Amazon.

These versatile pumps can be used in-line or as a submersible, in fresh or saltwater, and they're silent. Great for ponds, aquariums, filter systems, skimmers, fountains, water falls and water change systems. The small pumps (Mini and Mid) are suitable for running sponge filters and other applications for low-volume pumps.

External Aquarium Pump - Marine Depot

Miniature pump for small aquarium, regular watering, inflatable swimming pools, etc. Aerate and circulate your water with the Hydor Pico Evolution Mini Pump. This low profile aquarium pump is suitable for both marine and tropical setups. Its design also allows it to function in low water levels, making it a great addition to your terrarium. Keep your water oxygen-rich and moving with this energy efficient fish tank water pump. Keeping your water well-circulated is an essential part of maintaining a healthy aquatic environment. Moving water is more aerated, providing more oxygen for your fish and aquatic plants. In reef aquariums, circulating water mimics the movements of the ocean tides, stimulating invertebrate growth and health. Flowing water also helps support partially aquatic environments like terrariums for aquatic turtles. The Hydor Pico Evolution Mini Pump guarantees high performance circulation and aeration as well as total safety for your aquarium and terrarium life. This energy efficient fish tank water pump is ideal for tropical and marine aquariums, terrarium setups and much more. The small profile makes it a great nano aquarium pump while the very low water intake lets it work well in low water levels as in terrariums. The sleek, universal design lets you use the versatile Hydor Pico Evolution Mini Pump in a variety of setups. The aquarium pump’s flow control knob offers an adjustable water flow so you can customize it to suit your needs. Use the fish tank water pump to circulate water gently in a terrarium or create a petite power head for reef aquariums.

EcoTech Marine Vectra M1 Aquarium DC Water Pump.

This multifunction pump is ideal for use in tabletop fountains, small-to-medium size aquarium, hydroponic systems and water gardens. It comes with two nozzles (one with 13 mm and the other with 8.5 mm diameter) to use with various tubing sizes. It provides a maximum flow rate of 80 gallons per hour (300 L/H). The flow rate can be easily adjustable with a adjusting knob on the pump.

Best & Quietest Water Pumps in 2017 (REVIEWS) - Fish Tank Advisor