WIth a 120 you likely will need a reactor once you have a big lot of stony coral.
The evaporation rate for a 55 should be about a gallon a day, which is why we have open sumps and/or open tops. Putting kalk into the topoff water means that a gallon of kalk laced water is being delivered to your tank every day thanks to evaporation and that kalk (calcium powder) can feed quite a lot of coral. Kalk is good up to about 70 gallons, and then you have to start thinking about a calcium reactor for a stony reef.
So, it's not REALLY evaporation itself.. it's the fact the ATO kicks in and replaces the lost water with new water doused with Kalk?
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Sooooo much what you said I learned the hard way.
when I first got my tank, 13 years ago, I tried some of the "O, this looks like a cool coral!".. and they would die. (We didn't have these spiffy forums back then to ask/learn).
I found out in time, I didn't have the correct lights for "complicated" corals. I had fish, but no skimmer..etc etc.
I really did find out what I could keep by the method of "O, lets try this thing". Usually it died.
What lived, is exactly the things you talked about as being "easy"... the softies..shrooms and stuff.
I'm in the process of upgrading the equipment (pretty much replacing everything but the tank and the stand.)
For a good laugh, read my attempt at doing so here...
Smedlin's attempt to upgrade his tank.
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I'd like to learn more about "dosing with kalk" and "calcium reactors".
You talked about water quality and nutrients, etc.. but what about water temp? My tank seems to run a constant 82F. I hoping if I EVER finish my canopy and switch to LED's, it will lower the temp some.. but I'd still like to know more about how temp can affect corals.