The ocean is not a static environment. The water is constantly moving, and in doing so brings food to and removes waste from sessile animals. It moves the water past, through, and around the bacterial beds that aid the nitrogen cycle. Also, the churning of the surface waters caused by wind and waves provides gaseous exchange, basically trading nitrogen for oxygen. Therefore, water movement inside the aquarium is of tremendous importance.
To generate this water movement, we must employ a series of pumps and power-heads. A common rule of thumb regarding water flow in a reef aquarium is to turn over 10-to-20 times the aquarium volume each hour. That would mean for a 50-gallon tank, we would want a cumulative flow rate of about 500-to-1000 gallons per hour.
Now don't rush out and buy a 1000gph pump and think all things will be well. Water movement in the open ocean is chaotic, not a fire hydrant blasting constantly in a single direction. A 1000gph stream of water in a 50-gallon tank would just knock things over and wash trenches in your sand bed. A better setup would be to use four 250gph power-heads, a 400gph pump as a sump return, and a wave maker. "But," you say, "That's 1400gph! Isn't that too much?"
Glad you asked that chum. If you put your 4 power-heads on a wave maker, most likely no more than two will ever be on at the same time. Therefore, average water movement per hour would hover around 500gph from the power-heads alone. "Oh," you say, "that, plus the 400gph sump pump gives us 900gph of average flow through the aquarium." Not so fast there Buckaroo. Assuming that your sump is under the aquarium, as opposed to beside it or over it, your sump pump must push against head pressure, which can significantly reduce the flow rate at the output. Assuming you have to push the water up three feet, and make 3 90-degree turns to get it back in the tank, your sump pump is working against roughly 6 feet of head pressure. This could easily cut your output down to 250gph. "Wow," you say, "that is exactly half way between you previously mentioned high and low optimum limits!" Funny how that works out idn'it?
Make sure when you are looking for pumps and power heads, you consider a couple of things. Ask around and find out which manufacturers are said not to chatter on startup. That way, if you do add a wave maker, every time a power-head lights off, it won't sound like muffled machine gun fire. (I am partial to MaxiJets, but I am sure there are others that will behave as well.)
Also, with the sump pump, as well as the power-heads, make sure that they do not generate tremendous heat. There are going to be so many things generating heat in your tank that around August it will be hard enough to keep the heat steady and in an acceptable range without a chiller, and chillers can be costly.
So, a flow rate of 10-to-20 times tanks capacity is a good target, and it is best to accomplish this will a few small pumps as opposed to on big one. Lastly, a wave maker, either a commercial model or a few light timers from the Home Depot, would make a good addition. Your corals will respond favorably to it.
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