I have 3 that I have used 1 old API a new API and an older Seachem. They are all reading very close to each other. I have not had any problems with calcium since I started keeping a reef tank.
I would try a controlled test. Measure a new batch and then add a ca product to increase it and make sure your test kit shows the difference. I'd start there to make sure that is working because 300 is really low, makes me question the test.
What's your alk? Don't think we saw that. 2 scenarios I can think of, an imbalanced system not allowing the ca to rise or bad test kit. I'm not sure what else it would be. If you we're that unbalanced I'd think you'd see other issues.
What type of salt do you use?
Do you use RO water
"Measure a new batch" Have you mixed a new batch of salt water and tested it?
What was the calcium level before?
How do you add calcium?
What is the size of the tank?
I have seen this for many reasons
1. Bad test kits
2. Bad batch of salt mixture
3. Your calcium level was supersaturated and precipitated out. This reduces magnesium as well making increasing level difficult.
4. Magnesium levels can have an effect on some calcium tests.
5. Underestimating how much calcium is require to raise your tank level
OK I came home tonight checked my
MAG 1380
Alk 8
CA 320. up from previous day
sal 1.025
I think I may have miscalculated the amount of calcium I needed to raise it to proper levels.
Thanks for a the replies.
Gives me a warm fuzzy feeling.::
I just got my test kits out here to the medway house tonight, if you want to try a Red Sea pro kit for the heck of it to check your ca/alk/mg for backup readings. Pm me if your interested