lemme just say now that this is gonna be a long one, but i'm hoping that i can clear up some of the questions, and help releave some of the stress....
i would stay away from the plexy, and go with lexan (polycarbonate) if you are gonna go plastic.... (water permiation, yellowing, cracking, and general strength)
setting up a sump properly is really pretty easy. And your concerns of flooding are valid, but, heres the deal, with everything that everyone has told you above being true, the thing that you have to picture is that if you are using a hang on back overflow, really the only failure point is the u-tube, and that's only an issue if you consistantly have insufficient flow...
nothing comes out of the main tank untill the pump in the sump pushes water up to the DT(display tank) to cause its level to go up, then as that level rises, the water flows over into the inside box of the HOB overflow, this causes an imballance between the inside box and the outside one. this differential causes water to flow from the inside box to the outside one...
from there, the water falls via gravity to the sump... (in plumbing)
In the event of a power failure(personally i live out in the boonies, so this is a daily occourence) i have a UPS on my return pumps.... (i scrounged it outta a dumpster, cleaned it up, tested it, and put a battery in it for $20 and bingo)
when the power goes out(without a ups) the water in both the drain comming down from the overflow, and the water in the return tube both flow back down to the sump. Here's the trick, your sump isnt full when its operating normally... theres a couple gallons of "empty space" in the top, that way this power outage flow has somewhere to go and not on the floor.
Like was said above, you drill a one-eighth (1/8) inch hole in that return line, right where the water level in the tank hits it. This way when the pump stops, it can suck air, rather than back syphoning the water out of the tank.
With the way that i have mine set up, i only get 2 gallons of back flow in a power outage.. but i have 9 gallons of free space in the sump, so it never overflows...
now, back to what i mentioned earlier about the u-tube and enough flow. This u-tube holds its syphon suction all the time, or should. but, for example, lets say you have a 300 gallon per hour overflow, but your only putting 150 gallons per hour through it. What can happen in this situation is, that over time, air bubles will collect in the top of the u-tube, and will eventually buildup to the point of breaking the syphon, then you have a problem. If done correctly, the worst that happens is that you burn up a pump. if you have enough flow through the u-tube, (again with the 300gph example) if you had 275 gallons per hour flowing through it, then there is enough flow velocity to push these air bubbles out of the u-tube, and thus it becomes self clearing. this is how it should be done....
controlling the flow down to the sump is non existant.. no more water can come down, than you are pushing up with the pump, so... if you are pushing 275 up, then your gonna get 275 down... there is NO restrictions on the down hill side...
the tiolet float that i think you are talking about is for an automatic top off or ATO, that is something that some people add to thier sump to automatically replace evaporated water... i built mine from a dogs water dish that you could hook a garden hose to, and it would automatically fill the bowl when the water level got low, well, thats exactly how i'm using it.. when the water level drops in my sump(specifically in the return pump section) the float dropps, opens the valve, and then i have a small pump that is on a timer, and at specified times that pump is triggered for 2 min, and pushes clean water into that float valve. It works abso-lutely-freakn perfect.
something else that i was thinking about... your dimentions of 7x15x16 is a total volume of 7.27 gallons, roughly, subtract the power outage safety margin, and your looking at about 4 gallons of sump/fuge... thats not much, but it'd be enough to have some cheto and some sand in, and help clean up your system that way, but as for a real water volume adder, its something, but not a lot....
if you wanna talk about it, i can PM you my cell # and we can chat at length about it after i get home and get ready for tomorrow.... Just send me a PM if you wanna...
