Usually the issue is with the pin spacer. For thick glass on a tank your size the recommendation is to not even use the pin spacer. And since you said you are not using the pin spacer then you might as well send it back if it isn't going to stay. Not to knock your purchase but these powerheads seem to be a bit of a 50/50 hit or miss. Some people say they are the greatest while others have so many problems they end up returning them, sell them, or threatening to toss them in the trash. They have a reputation for quality control issues. If you ever check out the EchoTech Marine forum over on RC and read through the threads you'll find a plethora of people dissatisfied with the product and customer service. Sorry, probably not the answere you were hoping to hear. Just my observations.