Well, Alk does seem to be consumed from week to week.
Truth be told, I don't care about corals at all. But since I added them, its frustrating to not see colors. I only started with those bc they are easy and doesnt hurt my eyes. I'm an extremely busy worker and I'm not looking to add too much on my schedule, that's why I tried to DIY automate most of the things I can in the tank.
At first, even with that many fish, my refugium would keep phosphates and nitrates at zero. Clear water, clear glass, no dosing, nothing, and overall, happy, active fishies. I have a thick sandbed, more rocks under the sandbed and seachem matrix / fluval little tubes in the sump, so plenty of surfaces. I also have a 200 gal air pump in my sump, before just running inside air (outside air when hot) and now pumping thru an Co2 media, so yes, I don't have my powerhead pointed up bc with the fish guard, the Nero 5 won't tilt.
I was happy with the fish-only aquarium, precisely because I could keep the thing running smooth without messing too much on it. I lost count of how many times I tested the water at the LFS to see 0 nitrates/phosphates, tests that cost 15 a pop. I lost count after 150 bucks.
I started testing weekly when I started with Gsps. Everyone said that they grow like weeds and even grow in the "toilet". Then my LFS told me to keep my nutrients around 5-20 nitrates, 0.03 phosphates, around 1300 magnesium. I went and bought all Hannah checkers: Alk, Nitrates, phosphates, magnesium, PH, and salinity. Salinity I won't put it on the spreadsheet, it's always between 1.023 -1.027 and I think that's ok, even if stays in the low or high for a few days. I only do top-offs on Wednesdays and Saturdays. Not one individual told me that I have to maintain salinity at 1.026 all the time, otherwise nothing would grow.
Then I went to buy some stuff from a reefer, and he showed me all 3 of his tanks, growing, nice colors, and zero water changes. We believed that if my water parameters were within range, I just needed to dose what was lacking and change my water when something goes to **** or monthly or 2-3 months, just for some peace of mind.
I will be honest: I always went that route. I never tested doing water changes weekly in my life. I dont see myself doing water changes weekly for only a few corals - and corals that "grow like weeds". However, based on this post I will try for a few months to see what happens. If it helps great, if not, I rather give them away. If it doesn't work out I take the corals off and stay fish only or just give my whole setup away to my niece, who already has a FW system.