I tried this in another forum with not much success
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Do the plants/fish need water circulation during the day? I have a spray bar towards the bottom for CO2 return but do i need surface agitation during lights on? At night time i can't decide between air pump or powerhead to increase O2 levels in the water. What do you think? Which is preferred and why?
LAKA

OK i did a forum search in both GENERAL PLANTS and CO2 section.
It appears Tom has answered the above questions.
In a nutshell need surface water cirulation 24 hrs a day. During lights on helps dissipate excess CO2 gas and introduces O2 in tank as well (assuming atmospheric O2 concentration greater than O2 in tank water). At night time O2 introduced in water once again which is critical for fish to counteract plant respiration. Amount of CO2 in tank night time not critical.
LAKA

definatly need it on all the time. I find that I can put about 50% more CO2 in during the day without any adverse effects on the fish so long as I have quite a bit of surface movement. I was surprised as to how much difference it actually made.

One more question please regarding powerheads. My tank is 6ft long and i want to use a spray bar along the entire length just below the surface. Powerheads are rated by flow rate and "head height". Which is the more important? Does a higher head height mean a better flow of water against resistance? I note some powerheads have a high flow rate but not much of a head height.
LAKA

One way to look at it is the more head you have the more pressure drop your powerhead is seeing. That means more power is going to overcoming the head rather than pumping water.

A PH that has more flow at a higher head (say 1 ft of water for example) will pump more water than one with a lower flow at that amount of head.

There are 2 powerheads that i'm looking at. One pumps 3800 litres/hr with a head of 2 metres. The second powerhead pumps 3000 litres/hour with a head of 3.5metres. Which powerhead will have a higher flow rate through a 6 foot spray bar if the powerhead is situated level to the spraybar?
LAKA

Without knowing the pump curves for each of these it's really difficult to accurately compare them. The problem is that you have the flow for each one at different heads (resistances). Most likely the heads they list aren't the head you'll be seeing too.

This is a common trap by manufacturers. They list the optimum performance of their unit but don't give you the data to compare it to other manufacturers. It makes their product look good and prevents you from seeing that a competitor’s product may actually be better for you.

On the surface it sounds like the second one (3000 L/h @ 3.5m) may give you more flow at a given head, but it depends on the shape of the pump curve.

Here's a link to a nice, brief description of the relationship between head, flow, pumps, etc... Give it a read and it should help you a lot.

http://www.aquaticplantcentral.com/f...he-basics.html

I should mention that not all manufacturer's keep you in the dark. Ehiem publishes pump curves and Rio gives you flow readings at several head values. Others may do this as well, but not all of them I think...