@Sisterlimonpot
This person would probably know whats the best.
She seems like she has vast knowledge in 3D printing.
I know a little about a little.
PETG is hands down the ideal plastic for our aquariums.
PLA is popular because its cheaper. The issue becomes, most FDM 3d printers and slicers were designed around PLA. Which means print quality is always best with PLA.
PETG is 2nd (in my book). Thermal expansion isn't bad and most bed materials can keep the part from warping, peeling and/or popping off.
When you start getting into the exotics like abs, nylon, pom etc, the challenge becomes keeping the plastic warm enough that it doesn't shrink. Ie, the layers close to the bed stay warm but as you add layers those start to cool faster and warp, thus tugging on the printed part encouraging warping or worse the part not sticking to the bed. It's ideal to put the printer in a heated enclosure and regulate heat at that point. For that reason I stay away from those filements.
Another good filiment is the flexible plastics like TPU. That is reef safe as well. And prints similar to PETG.
PLA, does break down in salt water. Although I don't have much experience with it and never tested it long term.
I may be talking out my rear, but as I recall it being explained to me, PLA breaks down because bacteria feeds off of it in the same way a bio pellet reactor works (if anyone remembers those bio pellet reactors for controlling nitrates, they were popular circa 2010).
I don't know if you can label PLA as NOT reef safe. If people are having issues, I would think it's more about releasing the dye as it breaks down. Reds, and yellows are the colors to stay away from in that regard.
Anyway, I'm of the opinion that PETG gets the job done in just about any project and when you need something flexible, TPU is also your friend.
As far as glue sticks go. I use them sometimes. I couldn't speak smartly on weather or not it's reef safe. The great thing about glue sticks is the glue is water soluble, so you can rinse you part in the sink and it comes right off.