No problem Ben.

First off we need min and max temperatures. Lets go with a low of 68F and a high of 85F. We can adjust these temperatures as needed.

Now, 1 btu is the energy required to heat 1 pound of water 1 degree. You have 210 gallons and a gallon weighs 8.35 lbs. So, you have 1750 lbs of water as you mentioned before. So, if we heat water from 68F to 85F, you have stored:

85 - 68 = 17

17 * 1750 = 29750 btu

I don't know about your house, but in the worst of winter, my house looses 450,000 btu per day. To get this I just took my worst gas usage in therms and divided it by the number of days. Then I multiplied the therms times 1000 to get BTUs.

So, this would provide me with roughly 6% of the heat required for my home. Dang, I really need more insulation.

Hopefully yours is better!

If we wanted to get more heat out of it, we could heat it up farther of course. Lets say you take the fish out and are now able to heat it up to 130F.

130 - 68 = 62

62 * 1750 = 108500 btu

Now we're talking some pretty decent heat. Of course, you also now don't want to even touch the tank as its HOT.

But, thats what you can do with 210 gallons of water.