Don't understand test results

[Heatherabel3]

Can someone help me understand how all of this works together and why, whatever is going on, is going on.
On 5/10 my Alk was 10.5, Ca was 600 and Mag was 1340
We did a water change (28g nano) and today Alk is 8.4, Ca is 440 and Mag is 1460
pH is consistently 8.4 ish, it's kinda hard to tell exactly on the API test kit.

 

[disc1]

How big of a water change? What is the alkalinity, calcium, and magnesium in the fresh saltwater you added?

 

[Heatherabel3]

We took out 7 gallons. I honestly have no idea what the others were in the fresh saltwater we added. I never even thought to check it.

 

[bertoni]

That's a large chunk of water for that size tank. My first guess is that the water for changes had different parameters, but getting accustomed to all the various test kits can be an issue, too.

 

[Icefire]

even if you was the water change the make up water would have to be 0 Calcium...
75% * 600 ppm + 25% * 0 ppm = 450 ppm

Unless you added RODI instead of saltwater it's impossible.

I suspect bad testing

 

[Heatherabel3]

I am using the Red Sea test kit for Ca, Alk, and Mag. I wasn't as concerned with the Ca as I was with the Alkalinity. Is it okay to drop that much that fast? And we def added saltwater.

 

[bertoni]

A 28g nano might only have 14-15 gallons of water, depending on the situation, but I agree that testing accuracy probably is an issue here. I would test a few freshly-mixed water batches, and get second opinions on the test kits.

That alkalinity change should be safe enough for most animals, although touchier stony corals might object, particularly if the level drops more.