Abbot,
I have a question.. once the tank is full whats stoping the "fresh" seawater coming in the fill pipe going straight out the overflow?? to me you could have stagnant water build up in the bottom of the tank.
Why ddn't you put the fill pipe in the bottom so that good water enters the bottom and old water flows out the top giving a flow through the tank at all times. if you were worried about the tank draining back through the filler pump you could have put a cheap plastic ball valve in the filler line!
I would just swap the fill and overflow lines over and put the overflow out the top over the side with a hose (no bilge pump required). Also you will need the overflow to be at least 25% bigger than the inlet so as the tank does not overflow!
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