I'm in Southern Ontario Zone 5b (Niagara escarpment)

I have tomatoes in pots, two plants per pot, staked. Pots are 12" x 12" x
12" deep. A bit of a gravel bed on the bottom, maybe 3/4" thick. They
face south and get 7 hours of sun a day. Soil is triple mix topsoil with
1/3 peat moss, and drainage holes at bottom of pots. These plants were
planted by children in a school then we inherited the plants when school was
out for summer. They flowered nice.

The bottoms of some of the tomatoes have gone black. A quick Google search
suggests this is prolly due to lack of calcium but we have 25 hardness hard
water. Our garden water is not run thru the water softener.

How can I determine if it's calcium defiency or not? What other
possibilities can this be? The leaves never wilt and growth and foliage
seem healthy. No bugs are eating the leaves. I've never grown tomotes
before. Our herb gardens and flower beds are fantastic and are watered at
the same intervals as our tomatoes.

If somebody wants pictures I can through up on the net if so.