Cyno can be a pain but not hard to get rid of. Its all about limiting nutrients when it comes to any form of algae which cyno partly is. Once nutrients are available then algae will grow as it uses it as a food source. The fact that you didn't have much success with a macro may partly be because it was out competed by another algae (possibly the cyno) for the available nutrients in the water column. When I had my cyno and hair algae out break, I harvested it in my sump where I made conditions more favorable for it to grow there rather than the main display and eventually it grew only in my sump and no where else. Each week, I removed most of it which exports all of the bound up phosphates, nitrates etc that it used for its growth out of the system and i just allowed it to continue to grow back. Eventually as the tank matured it grew less and less until eventually it stopped growing altogether and I was able to remove the light from my sump and never had algae issues thereafter. It's all about finding that balance. You limit nutrients and your on your way to getting rid of algae. I saw mention of rubble in your sump and would have to agree, it can cause water quality issues because as the rock sheds, the detritus will just accumulate in there shooting up nitrate and phosphate levels so personally I wouldnt use it. Liverock would be better served kept in the main display IMO. In the sump, it would require additional maintenance which isn't necessary.
As for the C-Balance, I use to use it to dose my tank. the thing with it though is you have to test regularily to use it. It's hardly ever a 50/50 dosage between the two bottles and I'd run through one bottle a lot quicker than the other. Also, as you add in more corals or some start to grow, the demands for alk and calcium change so you have to adjust accordingly. Same with removing corals so if you aren't testing your water regularly I wouldn't dose either of the two to be safe because you won't know what is going on.
Lastly, when it comes to corals, you want a matured system which comes with age. The tank has to find it's balance and the fact that you are having issues with cyno means water quality isn't optimum just yet. You can do a test for nitrates today and read zero and think hey! I have no nitrates but it wouldn't be an accurate reading. Any time you have algae growing, nitrates are present as it uses it as a food source for it's growth so you end up getting a false reading because the nitrates are bound up in the algae, but the fact that the algae is still in your tank means it's not doing you much good so you need to export it by manually removing it. Over time as the bacterial colonies multiply, your nitrate levels should drop and the algae should subside, but it takes a bit of effort on your part not to over feed the tank, allow waste, un eaten fish food, detritus etc to accumulate in the tank because all that will do is shoot up nitrate and phosphate levels fueling algae to grow.
Just a few thoughts for starters. I'd hold off on corals for now and get your water quality in check first. It took me one year to add my first coral so it does take a bit of time some times.
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