In most aquariums the filter provides water movement in the tank.
Water flow inside an aquarium is important for many reasons.
Here are the main ones:

Aquarium filters are rated for a certain size range of aquariums.
Some aquarists feel these ratings are exaggerated in order compete against other aquarium filter manufacturers.
Many aquarists want a lot of water flow (gallons per hour) from an aquarium filter.
The idea is the high flow rate stirs up solid debris and brings in into the filter media instead of the gravel.
This prevents sludge build-up and maintains water clarity.
Aquarium filters with higher flow rates often use larger filter cartridges or have more room for lose filter media like activate carbon, phosphate remover and foam pads.
While the increased filter media and flow capacity is desirable, sometimes the flow rate created by the more powerful filter is too disruptive for the aquarium.

Defining “too much” water flow is very subjective.
It all depends on your particular aquarium.

In many cases it is not the flow rate that is too high, it is the flow pattern that causes problems for the fish and plants.
In other words:

You may not have to give up the benefits of being able to use a large amount of filter material and a high flow rate.
First, try these techniques of flow redirection.

Rocks and caves act as buffers, redirecting strong water flow patterns.
Try adding or rearranging natural or resin caves and structures in a way that reduces the impact of a powerful water flow pattern.
In many cases your fish will find quiet havens where they can float with little effort.
Solid structures help to quiet the overall turbulence in the aquarium.
Driftwood also adds flow-diverting structure and creates slow-flow regions.

Plants provide quiet zones for slow fish.
Some aquarists pack their tank with plants.
Others create aquascaped zones alternating between open swimming areas and planted refuge areas.
Broad-leaf plastic and silk plants require no special care and come in a variety of colors and textures.
Live plants require more care and are best suited for dedicated planted aquarium set-ups.

Hang on the back (HOB) external power filters are designed with a flow-restricting valve on the filter’s intake tube.
By turning the flow control knob, water is restricted from flowing into the filter, reducing the output of the filter.
Filter adjustment is different than previous methods.
It reduces the gallons per hour flowing through the filter.
Water circulation in the aquarium will also be reduced.
This is the easiest way to cut back on strong water currents.

Smaller, less aggressive fish will find it easier to feed when the current is not as strong.

Canister filters sometimes include a flow control valve for restricting the flow rate.
Flow control valves don’t reduce pump motor speed.
The water pump continues to work against the restriction. Fortunately, canister filters use magnetic drive pump motors.
There is no concern for pump damage or over-heating as long as there is some water flow through the canister.

Sometimes the problem is not too much water flow, it is where the flow is directed.
If you’ve got a powerful canister filter of return pump, a single water outlet can create an abrasive water flow in one spot inside the aquarium.
In this situation the flow is not even throughout the entire aquarium.
Some areas have too much water flow, some don’t have enough.

Some canister filters come with a special outlet nozzle that splits the water flow in Left and Right directions.
Add-on flow diverters are also available to split the flow into two areas inside the tank.
If you’ve got a sump and return pump, you’ll have to assemble your own return manifold system.
By using flexible or ridged plastic pipe, you’ll be able to customize a manifold system that puts the water flow exactly where you want it.
Some aquarists add valves to control the flow from each return nozzle.
Custom manifold systems are popular on larger reef and freshwater aquariums that use a sump filtration system.

While the manifold system spreads out the water flow into the aquarium, recirculation takes some of the water flow and recirculates it within the sump.

Let’s say the water pump moves 200 gallons per hour but you only want 150 gallons per hour in the aquarium.
By adding a “T” and valves, you can send 150 GPH to the tank and divert 50 GPH back to the sump.
Recirculation does not put any backpressure on the pump like a restrictor valve would.
The pump is not fighting against the single restriction.

One of the hottest trends in reef-keeping are DC-current return pumps.
The DC motors allow for very precise speed control.
This makes it possible to turn down the water pump to achieve the exact water flow you want.
DC return pumps come with pre-programmed flow modes.
The flow modes are presets that create slow water surges, strong water flow and even timed periods of no water movement for feeding fish and corals.
DC pumps are used with filter sumps (internal and external) or as a stand-alone recirculation pump.

Most aquarists choose a filter that is closely matched to their aquarium size.
In some cases, we oversize the filter by selecting a model that is slightly more powerful than we need.
If your tank is experiencing too much flow or want to redirect water movement for more even coverage, just follow these helpful suggestions.
Reducing the water flow has never been easier with today’s adjustable aquarium filters and return pumps.