The exact wattage is dependent on water volume, tank surface area, ambient temperature, and convection (air flow), desired temperature (the warmer the greater the heat loss), and heat already contributed by things like pumps.
The general recommedation that I have gone by is 3-5 watts per gallon, so 570-950 watts for a 190 gal. Maybe get a pair of 300's to start. If they seem to be running all the time then perhaps get a third.
Yes you can put them in your sump although putting at least one in the main tank can be helpful if your circulation pump fails.
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