It would appear, that Cessna's wet wing design aggravates this. All tanks hold some water. A random borescope check will show this, based on my experiences with tank inspection, design, repair, modifications, etc. For an apparent variety of reasons, this problem is very slow to surface, to alert the aviation community. Mr Powelson's comments are most instructive, and I tend to agree that in the absence of gross design errors, education and operating practices, plus a bit of isopropanol are the best solution here.
All tanks hold some water.
A random borescope check will show this, based on my experiences with tank inspection, design, repair, modifications, etc.