Is a 100 watt heater good enough for a 30 gallon? Do i have to get a new one? If so how many watts?

I use 50 W per 10 gallons so for a 30 gallon tank, that would be about 150 W (they don't make that size so you can combine two or go up or down in Watts depending on the temperature differential). How many Watts you need depends not only on tank size but temperature differential which is how many degrees F warmer you want the tank to be than the coldest temperature in the winter. My maximum differential is about 15 degrees F. If yours is more than that or about the same, I would get a 200 W heater. If it's less, than your 100 W heater would be plenty. Most heaters have a table on the back that gives temperature differential and tank size and then how many Watts you need.

ive actually decided to get a 20 gallon instead of a 30 gallon to make things easier.