For parts that will be submerged under water, I offer both PLA and PETG thermoplastic options. PLA is the cheapest option with many color selections. Two caveats people bring up against PLA is that it’s biodegradable (which means it has a finite durability, alas very long in most cases) and that some color pigments could be detrimental if leeched into the water column. PLA under our typical reef tank conditions rarely breaks down and if so it does it at an extremely slow rate. While no guarantees can be made, I have personally used black PLA in my tank for years with nothing but success (no bio-degradation, no leeching). It is up to you, the consumer, to weigh in the pros and cons and decide on the price you’re willing to pay vs. the material properties you want.
If you want to avoid uncertainty about color pigmentation of the PLA filament, you can order “Clear (transluscent)” parts which is PLA free of any pigments. I offer it on all my parts. You can also order PETG if you want a part that will most likely outlast it’s PLA counterpart.
For parts that will be outside of water, PLA is always the material of selection due to it’s countless pros.