75 watts is generally rated for 20 gallon tanks.
I have had luck with fluval edge heaters that are 25 watts that are preset to 78. The only complaint is that they are not adjustable and a bit weak if you house is cold. They are also one the smallest temp set heaters on the market. But it is rated for a 5 gallon aquarium so I really don't know if it will cook your fish in a 1.5.
That being said, I plan on getting rid of all of mine becuase I want the ability to raise the temp higher if and when disease strikes and they keep my tanks 1-2 degrees lower than I want. But I doubt you will find the right adjustable one for your size.
The other thing I have done is use seedling mat heaters (becuase they can get wet) when I have had multiple bowls. you can get a large and put them underneath, but you still need to watch the temp on those since they are not adjustable. Maybe put them on a timer where they gone on intermittenly during the day and stay on a night unless you have a temperature controlled house with air and heat. I found they raised my 2 and 3 gallon bowls about 5-7 degrees.
Alos if they are glass aquarium to put them all on one mat might be too much weight. Maybe you need a couple. I put my mat under a black velvet runner to hide it and it looked great.
Here is the link to the one I bought & a thermostat (which I never tried) for them:
With any heater recommendation use it only when you have a good amount of time to monitor them for a while. Even the best heaters when used correctedly can be way off. Some need to be set higher or lower than what they actually heat the water to.