optimal water flow in the aquarium

This really depends on the tanks dimensions, filter capacities, tolerance for visible hardware and density of plantings among other things.

Why not post some of the variables listed above and what plans you have already and then the collective braintrust here can discuss them with you and come up with a viable plan for your situation.

some background:

tank size: 40"X18"X24" (depth) (i.e. around 260 litres)
filter: Eheim 2222 (500 litres/hour) X2
lights: 4X39Watt T5HO at 6500K, 2XT8 32Watt, at 10000K and 14000K ...light on for 9 hours daily.
plants (all with high light demands) are planted moderately dense. My focus is on a carpeted bottom with cluster of different plants scattered around rocks placed here and there....
currently no significant trace of algae, except small patches of BGA on the tubling of the filter....they just sit there and seems not to scatter all around....
Others: ADA Amazon soil, PH~6.5, KH~2 to 3, I'm using ADA ECA and Brighty K, as well as Tropica's Tropical Master Grow....

With a 90 gallon tank you could put a powerhead on the opposite end of the filter return. That should kepp your water flow constant and keep debris off the bottom.

Do you really want that much circulation? I know you need some to clear away debris and stuff, but that aside I think you would want as low as possible? I hate it when my plants grow up to the outtake and starts swingen and turning. Cyperus and the like can look good, but most stem plants just like a tornade hit them... I guess the most common way to count this is by how many times you tankt circulates in a hour. I have a 1200L /h pump, and a 440 tank, so around 3 times a hours, and this is more than enough for my tank

Even flow is a key thing, not less flow.
The flow in rivers is even, not just one concentrated stream of flow.
Use spray bars etc.

Good flow helps bring nutrients, and CO2 namely to the leaves.

Plants wave around commonly in flowing waters in nature.

Regards,
Tom Barr

I use an Eheim 2224 on a 30 US gallon (136 litre) aquarium and that seems to be just right. The 2222 might be a little slow on a larger tank.

Andrew Cribb

Thank you everybody.
From the point of view of nurturing the Nitrogen-cycle bacteria (Nitrosomas etc) is it desirable to have a 'quick' flow or a 'slow' flow?

I imagine that every cannister filter is designed to work best when set to operate at more than 50% of its flow rate capability. If, for example, one had a large Eheim 2026 on a 30 US gallon aquarium, one would have to turn down the flow rate to 50% or less so that there was not too much turbulence in the tank. But if one had an Eheim 2222 on the same tank, one might well turn the flow rate up to 100%. With low flow rate, the water might not penetrate certain areas of the filter media. Higher flow rates would, presumably, push the water through much more of the filter media and, therefore, keep alive a larger bacterial colony. Both biological and fluid dynamics aspects of cannister filter technology suggest it is best to keep the flow rate above 50%.

Andrew Cribb