Why is my calcium so high?

[tableforglasses]

Looking for some help figuring out why my calcium is so high.

My 300-gallon reef (about 365 gallons total volume) has been consistently testing around 650 ppm calcium, and I’m not dosing any calcium, 2-part, kalk, or anything else.

Here are the details:

• Salinity: 35 ppt
• Alk: 9.28 dKH
• Calcium: ~650 ppm
• Magnesium: 1279 ppm
• Nitrate: 1.0 ppm
• Phosphate: 0.20 ppm
• Salt: Regular Instant Ocean — NOT Reef Crystals
• RO/DI water tests at 0 calcium
• Freshly mixed Instant Ocean at 35 ppt tests around 650 ppm calcium
• Display also tests around 650 ppm
• Calcium has been verified with both my Neptune Trident and a Salifert test kit
• With the Salifert test, I use the entire first syringe of Ca-3 and about another 0.10 mL before reaching the endpoint
• I’ve gotten similar high calcium readings from multiple bags of Instant Ocean
• I do roughly 20% weekly water changes

The tank is about 7 years old. I actually switched to regular Instant Ocean specifically because I was trying to bring the calcium down, but it hasn't moved much.
I have tested freshly mixed saltwater multiple times and it's also around 650, and I have tested unmixed RODI from my system and calcium is 0. I'm confused about where the calcium is coming from.

Has anyone experienced regular Instant Ocean mixing anywhere near this high at 35 ppt? Is there something about my mixing process or testing that I'm overlooking?

Any ideas on what I should check next?
Thanks for the help!

 

[Decamicron Labs Verified Sponsor]

Have you tried with a different batch or brand?

 

[Piotrek]

You alk is also high. Are you sure you are mixing and measuring correctly? Maybe you have an issue e.g with salinity meter and actually mix too much salt?