I think it depends on the coral. On my 29g tank, it only has some RBTA, some trash palys and some xenias in it. I do water changes on it about once every 3-6 months, maybe less. The filtration on the tank is just a HOB skimmer. It has GHA that just grows all over the tank, and about every 3 or 4 weeks I'll just pull it off as my nutrient harvest method.
I never dose that tank. I don't feed the anemones directly, but they catch a ton of fish pellets when I feed the 2 clownfish. So I'm sure they get a decent amount of nutrients from it. I don't test that tank either, been almost 2 years since anything other than a salinity test. Easy to keep, low maintenance coral and the overall system with only 2 clownfish has a pretty small bio load.
But my 180g mixed reef is different. I can tell big differences with trace elements vs none. But each corals is different in the responses and what they need. For example, many of them will still be ok and grow slightly without dosing. Zoas, leathers, acans to some degree, etc. However, other coral like my monitiporas will brown, their growth will slow, etc.
I posted in another thread about a coral I thought was dead. It was a cyphastrea. And by dead, I mean looked like it was part of the rock in color, no signs of life and remained this way for months. I didn't do any water changes, all I did was dose trace elements, and the entire thing came back to life. No idea which one it was missing, but it made a big difference. 7 months later and it's grown 3 times the original size.
And even in the cases where they weren't dying or turning brown without the trace elements, they would become pale and their growth would be much slower. With trace elements, the color difference is noticeable, and so is the growth rate.
