Is it possible to define low moderate and high flow. do i need to have a k3 blasting directly at any SPS I may get. I know my mushrooms prefer what i would consider almost no flow but is that low
is 2 k3's in a 55 moderate or high
how about in a 90.
it all just seems so subjective
Yeah... it is.
No one can really say if "x" number of k3s in a 55 is low or high... it all depends on placement and rockwork. If you have them pointing off into open water, then that's probably moderate flow. If you're bouncing them off rocks or blasting them into the glass, then that's obviously going to spread out the flow and reduce the water speed. Regardless of the amount of flow, no coral is going to want to be constantly blasted point-blank by a powerhead. It's best to set up your powerheads so that you have areas of high flow, areas of moderate flow, and areas of low flow. Then place your coral accordingly.
Unfortunately, that means a LOT of planning. You have to get a feel for what corals you want, and where they want to be lighting-wise. And then based on that, you can figure out where those high/medium/low flow areas want to be.
Regarding what low/medium/high actually is, Eric Borneman has a somewhat useful chart in his book "Aquarium Corals". (I highly recommend it!) While most powerheads are rated in gallons/hour, what you're really after with flow is what the velocity (speed) of the flow is. Here's Eric's opinion...
Low flow: 0.5-2 inches/sec
Medium flow: 2-8 inches/sec
High flow: 8-20 inches/sec
Very high flow: >20 inches/sec
During feeding time, with all your pumps on, watch particles of food as they zip around the tank as you count off the seconds. That should at least give you a feel.
