Salt Solutions, first, I'd look at getting rid of that cannister filter or filling it with live rock. if you do it, do it gradually, so as not to hit your unprepared sandbed with a sudden glut of detritus. Cannisters build up waste, ergo nitrates, sometimes ammonia, and corals aren't happy with that. Live sandbed takes waste, breaks down to harmless nitrogen gas, sends up to air, bye-bye. The one rule i encourage is keep your fish load within the limits of your sandbed's filtration, no additional help if you can avoid it.
Corals are zero bio load unless dead, and actually help filter. YOu can pack a tank to the gills with coral.
However, and you may not be happy with this: an anemone in a coral tank is a time bomb. If something goes wrong, and it moves and gets stung by other coral, or stings other coral itself, WWIII breaks out, the nem turns loose, floats about the tank hysterically stinging everything, everything goes on the warpath with tentacles if it's got them, and non-tentacled coral just dies and starts a nitrate spike which takes out everybody, even the fish.
I would suggest moving the nem out and letting clowns set up shop in the toadstool.
Now here you have a dilemma too: duncans and that toadstool are apt to quarrel down the line. Running carbon will calm that down, but the duncans may never grow as they might in a stony-only tank---
Stonies get along with other stonies or the milder softies, like mushrooms, gsp. They have to have a calcium at 420, an alk at 8.3-9.3, salinity 1.024-6, and mg about 1300.. YOU need test kits and supplements for all those things. The CHEAPEST way to supplement stony corals is by topping off with kalk water, and that is best done via an autotopoff system: a pump with a sensor that tells it to shoot 2 tsp water into the tank. Kalk can be delivered from a 5g bucket of kalk powder (mrs. wages pickling lime, 5 dollars for 2 lbs) dissolved in ro/di water (you have to have ro/di to make this work well). Kalk can't raise the alk and cal, but it can hold it where you hand-dosed to set it, and will hold it faithfully until the evaporation rate slows to a crawl, until the mg runs out, or until the bucket runs dry. Ro/di automatically dissolves the right amount of kalk to be safe: it's what water does in the ocean, dissolving limestones to assure ocean water is exactly what results, given enough salinity. The only thing you then have to do is keep that bucket full, add more kalk powder now and again, and keep your mg up, which means testing weekly.
I know that's probably got your head swimming---but that's the way the big tanks do it. There's a step beyond that, called a calcium reactor. But if you want a lot of corals and have 100g or under, kalk is your friend. It's not dangerous in overdose, great to learn with before you move on to huge tanks with mega-corals.
What I would not do is keep that deadly triad, the big leather (toadstool), the nem, and the duncans. Most of all the pretty nem has to go back to the store, asap, for the safety of all. Then you really should replace the toadstool with gentler softies; or with stonies. And if you go the stony route, no mushrooms, only rare need for carbon, and keep the gsp off your structural rocks. Clowns love to hang out in euphyllias (frog and hammer) and will help feed them.
Lotta choices. But I think leaving that nem in there is a future problem that can take out the tank.
