Powerheads and Wavemakers

The oceans and seas of the world are never still—there’s always some motion in the ocean. In an aquarium, especially when keeping corals, proper water flow is essential for their health. Corals, being sessile creatures, rely on flow to bring them nutrients and remove waste. Additionally, good flow helps control algae growth, particularly cyanobacteria.

There are different schools of thought when it comes to water flow in a saltwater tank. Some aquarists prefer wave-like motion, while others opt for constant, steady flow. The best type of flow depends on the specific needs of the organisms in your tank. For simplicity, we’ll lump these two approaches together, as both can be effective depending on the species you keep.

How Much Flow Is Needed?

In saltwater aquariums, the general rule of thumb for flow is to aim for 10-20 times the tank capacity per hour. For example, in a 75-gallon tank, you’d want a powerhead rated for around 750-1500 gallons per hour (GPH). This ensures that corals and other marine life receive the necessary movement to thrive. However, freshwater tanks typically don’t need as much flow. Tropical freshwater fish, coming from calm lakes, ponds, and streams, are less tolerant of strong currents, making a powerhead in a betta tank a poor choice.

That said, some flow is beneficial for freshwater aquariums. While they don’t require the heavy currents typical of marine setups, a gentle movement helps maintain water quality and provides some exercise for the fish.

Finding the Right Balance

Too much flow can be just as problematic as too little. If your substrate is getting disturbed and won’t settle, or if your fish appear to be struggling to swim, it’s time to reduce the flow. Keep an eye on your fish and adjust the flow to ensure they aren’t fighting against the current. On the flip side, one of the easiest signs of inadequate flow in a saltwater tank is the appearance of cyanobacteria—slimy, stringy sheets in colors ranging from black and red to brown and green. These bacterial colonies thrive in stagnant water and can overtake your tank if not controlled. Increasing the flow will often help prevent cyanobacteria from taking hold, assuming your water parameters are in check.