What heater for a 190g...
 

Would a 250w heater do? Can I put it in the sump?

Thanks

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.

Always get atleast 2, incase one fails unexpectingly. that way you don't lose all your corals to a sudden temperature change of 10 degrees or whatever.

3-4
 

get 3-4 heaters if you can with the same reason as stated by fishmanty. Likelihood of 1 failing is more than 4 simultaneously. Unless you want to get an external controller like a RANCO thermostat with 1-2 300W heaters attached to be controlled.