I thought there was a thread a while back started by John, who wanted
recommendations for a heater in a large tank. But I googled and could not
find it.

The tank in question has a radius of 34" and a depth of 24", but I'll likely
keep the water at 18". This is roughly 200 gallons. I tried a couple of
heater calculators on the web, one said 28W (LOL) and the other said 700W. I
was hoping for less, but I imagine the 700 is close. Although the same
calculator said I need 300-400 watts of light for my 55 gallon. My poor
Cardinals would need sun glasses. I only want to raise the temp about 5°
F on average.