I’m not an expert on this, but I’ve worked with fresh and saltwater aquaria for years and here’s what I can say: food grade plastics should be fine to use, since they are not supposed to break down in water. I 3D print a lot, and PLA is not safe for aquaria because it is made of plant material and biodegrades in water over time, while PETG is generally safe for aquaria – but you have to be careful of what dyes/colors are used, same with ABS and nylon so long as they say ‘food grade’. Fish are pretty tolerant afaik, while invertebrates (especially crustaceans and cnidarians) are typically more sensitive to contaminants. That said, polypropylene (PP), and high density or cross linked polyethylene (HDPE or XLPE) are considered safe so long as they are virgin plastics (not recycled material). The surface and porosity makes a big difference in how long the plastics will last before biofouling – basically, microbes will form a biofilm and eventually other things will start growing on the surface, but periodic cleaning should keep this from building up. Also, keep in mind that each type of plastic has different tolerances to temperature, pH, etc., and the age of the plastic matters.

As an aside, there are micro/nano plastics everywhere. It’s unavoidable at this point. One of the biggest concerns for aquaria is other types of contaminants – other chemicals that are added to plastics (e.g., BPA) and heavy metals used in pumps, heaters, etc. Electronics and their plastic housings eventually fail, surprisingly often and quicker than expected at times, and the stuff that leaches out can cause a lot of harm to the animals, not to mention if a heater cooks the tank or just stops maintaining temp and the animals get too cold.