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If the OM can handle the 3500, I'd use the dart for that and the snapper for the return. Unless you're going to use all 4 holes for drain (or manifold the pump to feed your various reactors), I think 3500gph draw off of the sump may run it dry (and overflow the ... overflow). We found that we...
Is the sump going to be under the tank or in the basement? How much water does the Snapper push? Is that going to be the return pump with the dart on the OM4W? Is this essentially the 120 (but taller) footprint? I was hoping it was the 3' wide model! I see you haven't planned for actinic...
No. Salt is not creeping out without you knowing it. Chalk it up to a bad first reading and start adjusting. Also, I'm sure you know by now, but you need to top off with RO/DI and not salted water. Topping off with salt will creep the number up.
I have a calculator that I should be able to use for this (I created it when I was hypo'ing a tang). If you give me the exact salinity right now (what are you using to check it?) I can tell you how much water to change out and what the salinity should be of the change water.
One undersized heater will run and run and run, wasting electricity. One oversized heater could potentially melt your tank inhabitants in a malfunction. You're better off with 2 medium sized heaters, for overlap as well as redundant systems.