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What's the best way to dose calcium in a mixed reef tank? I've got a 75 gallon with LPS and SPS, running a protein skimmer and two powerheads. My calcium levels keep dropping below 400 ppm, even with weekly water changes using Red Sea Coral Pro salt. Should I go with a two-part solution or something like kalkwasser?

I've had good luck with a two-part system, especially for tanks with heavy SPS demand. Dosing pumps help keep things stable, but you gotta test daily at first to dial it in. What's your alkalinity sitting at? If it's swinging, that'll mess with your calcium uptake.

Kalkwasser in the ATO is the move if you want a low-maintenance option. Just make sure your evaporation rate is consistent, or you'll get wild swings. My 60 gallon mixed reef stays rock solid at 420 ppm calcium, 8 dKH alk, and 1350 mag with this setup.

Burst wrote: ↑Thu May 08, 2025 11:12 am I've had good luck with a two-part system, especially for tanks with heavy SPS demand. Dosing pumps help keep things stable, but you gotta test daily at first to dial it in. What's your alkalinity sitting at? If it's swinging, that'll mess with your calcium uptake.

Yep, dosing pumps are clutch. I run BRS two-part with a GHL controller, and my alk stays within 0.5 dKH of my target. But kalkwasser is cheaper if you're on a budget.

For a 75 gallon with mixed corals, I'd recommend a calcium reactor if you're serious about long-term stability. It's an upfront cost, but it'll save you headaches down the line. Just pair it with a good pH controller to avoid overdosing.

I've tried all three methods, and each has its quirks. Kalkwasser is great until your room gets too humid and evaporation slows. Two-part works but eats up dosing pump heads. Reactors are solid but need CO2 monitoring. What's your current maintenance routine like?

Stick with two-part for now. It's the easiest to adjust if your levels are off. Once you've got a handle on your tank's consumption rate, you can explore other options. Are you dosing magnesium too? Low mag will throw everything out of whack.

Before you change anything, test your magnesium. If it's below 1250 ppm, no amount of calcium dosing will help. I've seen tanks where the owner was dumping calcium daily but didn't realize their mag was at 1100. Fix that first, then reassess.

Thanks for the tips, everyone. I'll test magnesium tonight and look into a two-part system. My alk is usually around 8 dKH, but it does bounce a bit between water changes.

Good call on checking mag first. If that's low, your calcium and alk will never stabilize. Once you've got mag sorted, a two-part with dosing pumps will give you the control you need. Just start slow and ramp up over a week or two.