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Just a few random thoughts on the process you;'re going through.
You do not need mechanical filtration, that is, filtration that mechanically strains the water so that suspended particulates are removed. This removes much of the detritus and other suspended substances that make the mechanical filter turn into a nitrate factory as these same particles begin to decay in the filter medium. A good sandbed will process this detritus for you. Removing that detritus that accumulates in you rock and other protected areas with your water changes (siphon the rock and any little caches of detritus out and discard all the seawater, replacing it with your water change seawater) will reduce the workload on your sand bed and extend it's useful lifespan.
Using a Rubbermaid container is the cheapest (and prolly the most appropriate) means of constructing a sump/refugium. If your display tank is not bottom drilled, you will need to get an overflow that hangs off the back of the tank with a "U" tube. Make sure to get one of the types that holds a small compartment of water at the outside end of the "U" tube to keep the siphon patent. This is usually a small wall between the U tube and the actual drain to the sump that is as tall as the lowest level you want your tank to drain to in the event of a power outage (you can make this as high as the current water level).
Route the drain into your sump/refugium, arranging the drain so that it will not splash. The actual container needs to have high sides, and you'll possibly need to build a divider to go into the tank to slow direct currents down a bit to allow some of the air in the draining SW to bubble out. If you use a submersible return pump like the Mag 9, place a divider between the drains and the return pump that forces all the water both over an initial divider to strain out your refugium organisms (teeth at the top would be good) and under a divider (to remove any more air). These can be constructed of PVC and acrylic formed to fit inside the Rubbermaid, and do not necessarily have to be watertight, just snug enough to force the majority of the water to follow these paths. You will prolly need to make some sort of external structural support that will prevent the sides of the container from bowing out. A wooden 2x4 frame will do this, just make it the size of the container without any water in it, and put a pair of bands around the outside of the entire container (outside the support) to snug the Rubbermaid into the support.
Place your heaters in the area between the splashguard for the drain line(s) and the 2 dividers for the return pump. Prolly best to use some silicon for the dividers for the return pump if you use this area for your automatic top off. The level fluctuations will occur where the pump chamber is (without the dividers, it will be the entire sump), so this will be the location for your float switch if this is your means of automatic top off. If you desire to use an external pump, make sure to drill the hole for the bulkhead at an appropriate level, as low as you can place it, but not so low that you will not be able to install the bulkhead (remember it's internal lip). Use true union ball valves between the sump and the pump, and between the pump and the return lines to allow for easy removal of the pump for servicing.
The return line from the pump to the tank will need to have some type of distribution manifold, usually a piece of horizontal PVC pipe with holes drilled in it every so often. It would be smart to put some type of true union connector between the return line from the pump and your manifold, it will make both putting it in your tank and removing it for modifications or cleaning much easier. If you use a threaded connector between your "up and over the top" pipe and your horizontal pipe, you will be able to adjust the direction that your return current flows. Fill the tank and operate this to see exactly where your water line will be, then drill the siphon relief just below this level (small, about 2mm). Be careful doing this, as your tank will start siphoning through the return line as soon as you cut the power to the pump.
Make sure to start testing your system with the water just full enough in the sump to allow for the pump to run. see where the water rises to from this point when the power is cut to the pump. You can use this as a starting point to determine where you want the water level to be in your sump. Once you've established this, test the capacity of your system several times, waiting for at least 10 minutes to make sure that there isn't some little trickle that may ruin your floors. Keep in mind that your sump level needs to be high enough to account for evaporative losses (if you don't do auto-top-off), yet low enough so that the sump doesn't overflow in the event of the dreaded power failure. Personally, I would install a small lip around the entire base of your sump area in the aquarium cabinet such that it creats a pan, caulk all the joints/seams, and paint if with a marine grade epoxy paint to waterproof it. You'll thank me later
Set up your lighting so that your sump can now become a refugium, what you put there should be self sustaining and pretty much up to what you want for the refugium. Some shrimps, a small mix of mollusks (clams and snails are really good), macroalgae, misc. banned creatures that will not upset the balance (some crabs have found homes there, as long as they don't eat your cleanup crew), if you use a sand substrate, a cuke or two, and some extra rock is usually nice.
If you want a seperate refugium from the sump, a second containter may be set up in the sump area, located higher than the sump. Supply the incoming sw to this refugium with a small powerhead and allow the water to gravity feed back into the sump. If you use a drillable rubbermaid, you can place a hole in the side or bottom of the container and make an overflow inside the refugium that will drain back into the sump area (it can be as simiple as a piece of pvc pipe cut off at the level you want your final water level to be.)
HTH, this is the quick and dirty version, I am sure that others here will flesh this out for you.
Just a few random thoughts on the process you;'re going through.
You do not need mechanical filtration, that is, filtration that mechanically strains the water so that suspended particulates are removed. This removes much of the detritus and other suspended substances that make the mechanical filter turn into a nitrate factory as these same particles begin to decay in the filter medium. A good sandbed will process this detritus for you. Removing that detritus that accumulates in you rock and other protected areas with your water changes (siphon the rock and any little caches of detritus out and discard all the seawater, replacing it with your water change seawater) will reduce the workload on your sand bed and extend it's useful lifespan.
Using a Rubbermaid container is the cheapest (and prolly the most appropriate) means of constructing a sump/refugium. If your display tank is not bottom drilled, you will need to get an overflow that hangs off the back of the tank with a "U" tube. Make sure to get one of the types that holds a small compartment of water at the outside end of the "U" tube to keep the siphon patent. This is usually a small wall between the U tube and the actual drain to the sump that is as tall as the lowest level you want your tank to drain to in the event of a power outage (you can make this as high as the current water level).
Route the drain into your sump/refugium, arranging the drain so that it will not splash. The actual container needs to have high sides, and you'll possibly need to build a divider to go into the tank to slow direct currents down a bit to allow some of the air in the draining SW to bubble out. If you use a submersible return pump like the Mag 9, place a divider between the drains and the return pump that forces all the water both over an initial divider to strain out your refugium organisms (teeth at the top would be good) and under a divider (to remove any more air). These can be constructed of PVC and acrylic formed to fit inside the Rubbermaid, and do not necessarily have to be watertight, just snug enough to force the majority of the water to follow these paths. You will prolly need to make some sort of external structural support that will prevent the sides of the container from bowing out. A wooden 2x4 frame will do this, just make it the size of the container without any water in it, and put a pair of bands around the outside of the entire container (outside the support) to snug the Rubbermaid into the support.
Place your heaters in the area between the splashguard for the drain line(s) and the 2 dividers for the return pump. Prolly best to use some silicon for the dividers for the return pump if you use this area for your automatic top off. The level fluctuations will occur where the pump chamber is (without the dividers, it will be the entire sump), so this will be the location for your float switch if this is your means of automatic top off. If you desire to use an external pump, make sure to drill the hole for the bulkhead at an appropriate level, as low as you can place it, but not so low that you will not be able to install the bulkhead (remember it's internal lip). Use true union ball valves between the sump and the pump, and between the pump and the return lines to allow for easy removal of the pump for servicing.
The return line from the pump to the tank will need to have some type of distribution manifold, usually a piece of horizontal PVC pipe with holes drilled in it every so often. It would be smart to put some type of true union connector between the return line from the pump and your manifold, it will make both putting it in your tank and removing it for modifications or cleaning much easier. If you use a threaded connector between your "up and over the top" pipe and your horizontal pipe, you will be able to adjust the direction that your return current flows. Fill the tank and operate this to see exactly where your water line will be, then drill the siphon relief just below this level (small, about 2mm). Be careful doing this, as your tank will start siphoning through the return line as soon as you cut the power to the pump.
Make sure to start testing your system with the water just full enough in the sump to allow for the pump to run. see where the water rises to from this point when the power is cut to the pump. You can use this as a starting point to determine where you want the water level to be in your sump. Once you've established this, test the capacity of your system several times, waiting for at least 10 minutes to make sure that there isn't some little trickle that may ruin your floors. Keep in mind that your sump level needs to be high enough to account for evaporative losses (if you don't do auto-top-off), yet low enough so that the sump doesn't overflow in the event of the dreaded power failure. Personally, I would install a small lip around the entire base of your sump area in the aquarium cabinet such that it creats a pan, caulk all the joints/seams, and paint if with a marine grade epoxy paint to waterproof it. You'll thank me later
Set up your lighting so that your sump can now become a refugium, what you put there should be self sustaining and pretty much up to what you want for the refugium. Some shrimps, a small mix of mollusks (clams and snails are really good), macroalgae, misc. banned creatures that will not upset the balance (some crabs have found homes there, as long as they don't eat your cleanup crew), if you use a sand substrate, a cuke or two, and some extra rock is usually nice.
If you want a seperate refugium from the sump, a second containter may be set up in the sump area, located higher than the sump. Supply the incoming sw to this refugium with a small powerhead and allow the water to gravity feed back into the sump. If you use a drillable rubbermaid, you can place a hole in the side or bottom of the container and make an overflow inside the refugium that will drain back into the sump area (it can be as simiple as a piece of pvc pipe cut off at the level you want your final water level to be.)
HTH, this is the quick and dirty version, I am sure that others here will flesh this out for you.
