i didn't have anything specific in mind, but off the top of my head
1. how to drill glass and acrylic aquariums for overflows, what can and cannot be drilled, proper tools, speeds, etc.
2. in making calcium reactors, protein skimmers, things with combos of pvc and acrylic, how to glue, what to glue with, what not to glue with.
3. anytime your describing a project, what suppliers, shortcuts, things you have found that can make the job easier, save time, save money.
4. basic tooling for dyi aquarium projects.
if i think about it, i'll probably come up with something else. i think if you take it from the perspective of a neophyte who has not done dyi before and you are trying to teach him, almost as an apprentice, and build on that each month, the column will be valuable.
hope any of this helps.