Well, I look at it as a cheaper option. To me I see a closed loop using a Dart pump and 4-Way OM costing around $600-700 plus you have to consider plumbing and drilling costs which can make a total around $1000.
You can get a tunze kit with 2 pumps and controller for the same price but the 4 way OM provides 4 points of flow origin where as the tunze kit only 2. So really you need two tunze kits which costs double. So closed loops are GENERALLY cheaper for larger tanks.
Although some would argue you get way more flow from all those tunzes, try running a Dart at zero head next to a tunze and tell me which is greater. In addition the fact that you can use nozzles on a closed loop also creates high velocity streams which travel further and you with multiple nozzles pointed at multiple areas of the tank you eliminate more dead space.
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