cracked PP fitting

Bulkhead fitting failure

The intention here was to use the same fitting, above and below the water line. The tanks themselves were polypropylene (PP), so using a PP bulkhead fitting seemed smart. We ended up with 12 of these on our tanks, most below the water line. They are rated for the temperature, however they leaked.
At first it was only the fittings with pressurized lines attached. However, every year, when the tank temperature starts to rise in the Spring - another one starts to leak, even the ones with low pressure connections. I have spoken with installers who have not had problems. They may not have the same combination we had - high temperatures, household pressure (~60PSI) and tight fittings.

stainless steel bulkhead flange
A better solution is stainless steel. We have switched to a flange style bulkhead fitting, all stainless steel, with a Viton gasket. No leaks. We use this flange for PP, EPDM and PVC tanks or tank liners.