What heater for a 190g...

[fishmanty]

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.

[Chin_Lee]

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.