If you have an enclosed underbelly, you'll likely have to take down the plastic to get an eyeball on it. Check out your tank mounting system while you're under there. Many are chinzy. Mine was nothing more than a piece of 7/16" OSB board stuck between two floor joists. And it was half rotted from condensation. I replaced the support with a piece of 5/8" plywood with plastic spacers and weep holes drilled in for ventilation. Also added a few pieces of banding strap under the plywood for extra strength. Now the full water tank doesn't sag anymore.