The system for a sump as a few issues nobody has discussed yet. The first issue is that the overflow must have a max. flow rate greater than the return pump in the sump. As long as the overflow is not impeded, you will never overflow the tank. So you meed to have a design in which ot is not possible to empty the sump into the main tank beyond a given point. Then, what if the pump rails? the design mus insure that the tank can not be overflowed beyond the capacity of the sump.
The latter issue is easy to handle You place the overflow so that it stops before too much water can be allowed to leave the tank. However, the first potential problem needs a more complex solution. There are basically two options. One is to place the retunr pump high enough in the sump such that one a given amount of water gan be pumped out before the pump intake is above the water level. While this will prevent a flood, it will also likely burn out the pump from running too long with no water.
The other option is to use a float valve in the sump. This automatically turns water flow on and off based on a preset level. But this will cost up front. If the overflow clogs, when the water level in the sump goes down to a level too low, but not ebough to overflow the tank, the pump will be turned off. This saves the pump as well as stopping the flood.
Many years ago I bought 3 tanks used- 15o, 126 and 40L. I wanted to put altum angels in the deeper 150 but I have a bunch of bigger clown loaches going into the 125 and I wanted more water for them. So the plan was to do a sump with th e 40L under the 25. I planned to use the Poret foam sheets from Swiss Tropicals and do a design similar to the one posted above from the S. T. site. I consulted with Dr. tanner on the design. I bought an Eshopps overflow and a few other things and real life intervened. The clown loach tank at the time started to leak. So starting one Sunday morning and finishing in the wee hours of the next mornng, I cleaned the 150 which was filthy and then I moved everything from the 75 the fish were in and added more and then got the fish moved along with the canister on the 75 and the media from the AC 300 which went into a an AC 500 along with more new media. The sump never happened.
But the plant for it was pretty basic and easy to manage. The foam sheets start with a 10 ppi foam followed by a couple of 20 ppis ones. I was willing to risk losing the pump by raising it up off the bottom of the sump so the system was basic and very effective. The sheets could be removed as needed to be cleaned and return which would not be all that often. The water from the tank wpu;d enter the sump imn a wat as to roil the water surface and oxygenate it. This could be an issue with noise but not a great amount. A good stand could make it pretty quiet.
I did not want to drill the tank, so I opted for the overflow. Drilling is the better way to go if one can do it. No issues with getting the air out of the intake. Here is the one thing I do know. You cannot find a superior media than the Poret foam for use in filtration A different brand of a similar foam would do it. While I never did the sump. I have a bunch of Poret foam cubefilters and 4 Hamburg Mattenfilters. Thise are sort of a sump in the tank.
One final comment on Adler's sump design. it looks to ne for a SW tank as there is no live rock in FW. Some folks do what is called a veggie filter. basically this is a sump loaded with live plants which do most of the filtration. If one does this they have to be able to work on the plants in the sump and that means open space over the sump. If you are going to go with the Foam sheet design, bear in mind that, when the time comes to pull a sheet to cean it, you need enough room to get it out without making a huge mess everywhere. If it is any help, here is a pic of the two tanks I described soon after being put onto their stands, which I built in place.
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I later discoverd the prblem with this being a small room. The 40l had tp beslid into place from the end of the stand and I had to move the 150 to the left so I could slide the 125 to the left in order to get the under tank onto the stand bottom. Today that 40L holds a group of 21 zebra plecos and the 125 above it holds about 75 L173 plecos born here. The 150 old myc clown loach group. There are no sumps involved. *sigh*