Sounds like it might work all right, but let me see if I can follow what you are trying to do and why.
The float switch would be the exact same as you find in the back of a toilet. How often do you see those fail (personally, I have never seen one fail. I have seen some of the other stuff fail, but not the float switch).
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Of course they fail from time to time, have you never seen a toilet overflow working? That's because the float valve has failed.
1. I do not understand why you would want to do a 100% water change. For one it is not necessary. Two it is wasting water. Three it will cause more stress on the fish. Also you need to keep in mind, if for any reason your water company changes what they're doing to the water (say they double the amount of chlorine) you're filters might not be able to keep up and soon you have a huge problem on your hands (meaning you'll start having dead fish before you figure out what's going on). If you limit the amount of water coming into the tank, you might be able to catch major changes before a disaster. (that's not to say you couldn't have one even at 10% changes). I think at 100% per week you're going to spend too much money on the extra water without benefiting anything.
If you run it from RO, then the amount of chlorine is immaterial, as you have a filter to remove it. Just get an inline TDS moniter and you will catch anything before it goes wrong. Also, the more celan water the better.
I fail to see how a 20-30% water change a day (somewhere around 1-2% per hour) will be unduly stressful to the fish. Mine have no problems with a 40% once a week. It would be a constant flow. And after reading the results of experiments on other forums, you could run a system like this without any conditioner or RO once the filter colony is mature.
The main reasons why large water changes will cause problems is with a form of "old tank syndrome" where the stats of the water are materially different from the tap water. If you have a constant supply of freshwater then this will not happen.
I fail to see how a larger water change would prevent filters from coping, what do we always suggest when someone has a problem with the tank? Frequent (daily) water changes.
The best saltwater setups are ones with natural water being pumped in and back out on a constant nature with 100%+ changes per week. If you have to pay for the water then it will costs, but as I am on water rates I would not have to worry and would far rather have cleaner water. The solution to pollution is dillution.
2. I don't think I'm following your design intent. I think my plan is somewhat like what you're talking about, but instead of having a physical sump, I would have the incoming water coming from the filters (instead of from the sump pump) and the waste water (through an overflow box) would be connected to a sink drain (instead of returning to the sump).
I don't follow how you're going to get any water out of your system (if you add water you have to take away water or your tank will overflow). Can you do a drawing or describe it a little more?
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That is exactly what I described. An overflow to a chamber that is connected to the waste. As water is added, the wter level rises above the weir and then into the chamber tht is connected to the waste pipe.
Then, the only thing governing the levels would be the rate of supply of RO water. If the RO went mad and produced faster, there is no risk of the tank flooding (unless it goes mad and overpowers the drainage system, though 25mm can easily handle 2,000LPH).
You could either do it as a single tank, with the overflow to waste in the main tank, or do it with a sump with the overflow to waste in the sump tank and the water going into the sump. If you are determined to use float valves, then you could place them in the sump, and at a higher level have the overflow as a safety precaution.
Reef Central has a large number of stories of people who trusted float switches who now have very well soaked floors. If it can go wrong, plan for it to go wrong.