The short answer
Print in-tank parts in PETG. Use ASA for parts in a sump or under strong lights. Keep PLA for test fits. That’s what ReefCAD assumes by default, and it’s where most reefers who print their own gear end up.
PETG, ASA, ABS and PLA compared
Heat tolerance below is the glass transition range on typical datasheets, the point where the plastic starts to soften. Every brand publishes its own number, so check your spool’s technical data sheet. A reef tank usually runs far below all of them, around 24–27 °C (76–80 °F).
| Filament | Good for | Watch out for | Softens at |
|---|---|---|---|
| PETG | Most in-tank parts: racks, jails, holders | Strings a little; dry it if it’s been sitting out | about 75–85 °C |
| ASA | Sumps, lids, anything under strong light | Warps without an enclosure; ventilate while printing | about 95–105 °C |
| ABS | Out-of-water parts in the stand or sump area | Warps; less UV-stable than ASA; ventilate | about 100–110 °C |
| PLA | Test fits and prototypes, not the tank | Breaks down slowly in water; least heat-tolerant | about 55–65 °C |
PETG
A glycol-modified cousin of the PET in drink bottles. It’s tough rather than brittle, takes on very little water, and prints on an open-frame printer. That combination is why it’s the usual pick for parts that sit in saltwater all day.
ASA
Made as a weather-resistant alternative to ABS, so it holds up to UV and heat. It shrinks as it cools, so it prints best in an enclosure, and it gives off fumes while printing: ventilate the room.
ABS
Similar to ASA in strength and heat, but it yellows and gets brittle under UV over time. Fine for parts in the cabinet; ASA is the better choice under lights.
PLA
The easiest filament to print, and it won’t melt at tank temperature. The problem is the long run: PLA breaks down by hydrolysis, slowly at room temperature and faster when warm. A rack that has to hold corals for years is the wrong job for it.
What to keep out of the tank
- Metal-filled filaments (copper, brass, bronze, iron). Copper is toxic to corals and invertebrates, and you can’t know how much a print would release.
- Wood, carbon-fiber and other filled filaments. The filler is exposed at the surface and the additives are rarely listed.
- Resin prints, unless fully washed and cured. Uncured resin is harmful to aquatic life.
A “reef safe” label on a spool isn’t a tested standard. Plain PETG or ASA from a brand that publishes a data sheet is the safest bet.
Print settings that matter
For tank parts, how you print matters as much as what you print with.
- More walls. Three or four perimeters make a solid shell, so water can’t work into the infill.
- No trapped air. A sealed pocket of air makes a part float until it waterlogs. ReefCAD’s legs are open end to end and its sockets vent out the top for exactly this reason.
- Small parts, solid infill. For a plug stem or a peg, 100% infill costs a few grams and leaves nothing to fill with water.
- Calibrate in the filament you’ll use. Each filament prints holes a little differently, so print the fit-test coupon again when you switch. More in the plug size guide.
Before it goes in the tank
- Trim any strings or blobs, especially inside holes where a plug has to slide.
- Rinse the part in fresh RODI water to wash off dust and oils from handling.
- Check it sinks and sits the way you expect before you load it with corals.
Ready to print? Start with a frag rack, a coral jail or a probe holder, or follow the frag rack walkthrough.
