What you have drawn wont work. For an over the top overflow to work it must always be full of water. Any air gets in and it breaks the siphon and it stops. The tower will not stay full of water so you would have to build a cup in the top of it that does. The water can simply overflow that down onto the rubble. Or drill the tank because that is the most foolproof way get the water out.
What you have pictured there is a really complicated wet and dry filter. Most dont use those anymore either.
It looks like something used in a LFS that has nothing in the tanks with the fish so they can be easily netted. In that case they are using it to do the same thing we do with live rock inside the tank.
The refugium inline is fine until your macro self destructs one day, something they do for reasons no one understands mostly. Then it plugs up your return pump. I have a remote one with it's own drain and pump and a water level sensor that controls the pump.
I like chemicals. Literally everything you put in the tank from the fish and everything else is made from them. There are good ones and useless ones.
There is no less helpful word to a new person than natural. It implies so much and tells you nothing.
There is nothing natural in a reef tank. A real reef is connected to the whole ocean. An infinite source and sink in most ways. We have no effective way to copy that. So we add stuff that could be called chemicals and remove stuff that could be called chemicals with other chemicals.
So what would a natural system look like. Dead stuff in the ocean is degraded one way. Something eats it. This happens all the way down the food chain. The next level down eats what the ones above them produced. At the end are bacteria of 2 types. Aerobic and anaerobic. Aerobic is easy in a reef tank. Every surface that is hit by the flow from the wave makers grows bacteria.
It is the anerobic part that is tricky. It will only live where oxygen cant get to. Usually because aerobic bacteria used all the oxygen up in water flowing by. The center of a rock or the very bottom of a deep sand bed. It is difficult in a typical tank and stand contained filtration system to get a usable balance between the two types. But the anerobic bacteria are the ones that finally take nitrates and reduce them and make nitrogen gas and send it back to the atmosphere. That is the completion of the natural way.
So reef people do water changes and dilute the nitrates (and phosphate) out of the system. Or use a reactor containing sulfur that grows anerobic bacteria. It removes nitrates only.
Or perhaps you should look up what a Jaubert plenum is. This creates an anerobic area in the tank below the sand. I have never made or used one but might be exactly what you are looking for.
and a biopellet reactor. (grows anerobic bacteria as a crop that I removed by a skimmer) This removes both.
Or a sulfur reactor. And algae scrubber will do more for you than a refugium.
The two final products of a tank food chain are nitrates and phosphates. You have to test the levels of them and have an effective plan to deal with them.
Nitrates are somewhat easy to get rid of. Phosphates less so.
I use a phosphate remover. That is considered high level black magic these days. A chemical. It forms a precipitate in the tank that MUST be filtered out. I also have a sulfur reactor. That is how I run my system. There is also GFO. A media that will remove phosphates but can become an expense and a maintenance chore.
Frequent water changes are easiest with a small system and chemical remediation easiest with large ones. With tanks in between you have to decide what you want to do every week to keep it going.
It is way easier to keep a tank with a few very interesting things than one jammed full.
Purchase Live Marine Specimens -
Thats enough I wont have to type the rest of the week.
