Normal alkalinity but low calcium

[Buckyfan]

This this tank is a downgrade from a 40 gallon tank with a sump. The whole system had been set up for at least a decade. Just having problems with my calcium being low. Attached is the most recent chemistry test from the local fish store. As you can see in the picture I don't have much to use up calcium in the tank but the calcium is always low. I have tested it six or seven times and it's always in the same range. The calcium at 1.024 was 409 in my water change water before I did the water change. This new tank has been set up about 3 months and I've been doing 10% water changes about weekly. It just doesn't seem like I should need to dose calcium in such a small setup. I travel fairly often which is why I downgraded the whole system. It's not super practical for me to be dosing something daily. The corals that are in there are all new but they all seem to be doing well.
Do I have to start dosing? If so can I dose calcium without affecting alkalinity? I've never dosed before so I'm not sure how to do it?
Is there something I'm missing that I could just change as far as my husbandry? If the water change water is 409 where is the calcium going?

 

[Randy Holmes-Farley Verified Community Expert]

I think test error is a likely explanation. Was the 409 and the 298 ppm Ca using the same device?

Have you been dosing alkalinity only?

 

[Buckyfan]

Yes those were both the same device. It's a chemistry analyzer from the local fish store. I suppose it could be off. I have been dosing nothing and never have. When I test it at home with my red sea calcium kit I also get low numbers. So low that it won't even register on the titration but that is a really old kit so I don't trust it.