As a 15 year old kid (1990) I was the first employee of a woman opening a LFS. We set up 60+ aquariums.. every single tank had an under gravel filter. And it worked!
Is there a benefit to having a third 100 watt heater in a 125? One of the 100 watt heaters is heating the tank by itself. The 2nd heater doesn’t even turn on. I keep the thermostat in my house set at 72 winter and summer.