Ruth,
If one of the two closed loops pump leaked, the water would go directly into the sump, as well the water level would only drain to the first hole drilled on the closed loop which would act as a siphon break. The sump is also drilled with its own overflow so should the water level in the sump rise for any reason it will go straight to a drain in our basement.
One of the nifty features of the posidon is that if they run dry they will shut themselves off. I'm not aware the the dart pumps offer a similar mechanism.
Heat isn't an issue, the tank will be properly vented and a approximate 4 degrees of heat from pumps would only serve to reduce the workload on the heaters we've installed. It might be an issue had the tank been smaller, but with an approximate water volume of 300g its more of a helping hand then a hinderance.
Thanks for the offer of the lights Ruth, but we're going with 3x400w MH. Hope you find a buyer!
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