Filter feeders are the canaries of a reef tank. When they're thriving, your water quality is right and your nutrition profile is broad. When they're closing up, dropping mantles, or starving slowly, something fundamental is off.
This guide covers what each major filter feeder group actually eats, and the dosing habits that keep them healthy.
What "filter feeder" actually means
A filter feeder pumps water through its body and extracts food particles from it. Different species filter different size ranges:
- Clams. Filter dissolved nutrients, bacteria, and small phytoplankton (1 to 10 microns).
- Sponges. Bacteria-sized particles primarily (0.5 to 5 microns).
- Sea squirts (tunicates). Phytoplankton and detritus (3 to 50 microns).
- Feather dusters and Christmas tree worms. Slightly larger plankton (5 to 100 microns).
- Gorgonians (the photosynthetic ones aside). Mid-sized zooplankton and phyto (10 to 200 microns).
Most filter feeders prefer a continuous trickle of food rather than feast-or-famine dosing.
Tridacnid clams
Tridacna clams are part-photosynthetic, part-filter-feeder. They host zooxanthellae like corals, but they also actively filter the water for nitrogen, phosphorus, and small particulates.
What works:
- Strong, full-spectrum light. Non-negotiable. Most clam deaths trace back to inadequate lighting.
- Live phytoplankton (especially Nannochloropsis). Dose 5 to 10 mL per 100 L every day or two.
- Stable nitrate (5 to 10 ppm) and phosphate (0.05 to 0.1 ppm). Clams need some nutrients, not zero.
- Calcium and alkalinity in range. They build heavy shells.
Common mistake: starving the tank in pursuit of "ULNS" (ultra-low nutrient systems). Clams suffer first.
Sponges
Sponges are bacterial filter feeders. They prefer continuous dosing of:
- Live phytoplankton (small-cell strains like Nanno or T-Iso). Indirect benefit via the bacteria that bloom on phyto by-products.
- Bacterial supplements. Products like MicroBacter or homemade alternatives.
- Detritus and dissolved organics. Sponges thrive in slightly nutrient-rich tanks.
Most sponge deaths come from exposure to air during transport or aquascaping, not from feeding mistakes. Once an air bubble enters a sponge, it usually rots from the inside.
Sea squirts (tunicates)
Sea squirts are dramatic-looking and notoriously hard to keep long-term. They need:
- Heavy live phytoplankton dosing. Daily, in larger volumes than clams.
- Stable, nutrient-rich water. Like sponges, they don't tolerate ultra-clean tanks.
- Low to moderate flow. Strong flow tears the soft body.
- Skimmer off after dosing. Otherwise the phyto disappears before they get any.
Most home reef-keepers find sea squirts difficult. Plan to dose more phyto than you think reasonable, or accept that they'll waste away in a few months.
Feather dusters and Christmas tree worms
These tube-dwelling worms are easier than clams or sea squirts. They want:
- Live phytoplankton. Standard reef dosing rate is enough.
- Live copepod nauplii or rotifers. Excellent for the larger feather dusters.
- Calm flow at the crown. Strong flow makes them retract constantly and stop feeding.
Gorgonians
Gorgonians divide into two groups:
- Photosynthetic gorgonians. Treat as a soft coral. Light feeding and good light is enough.
- Non-photosynthetic gorgonians. Daily live food and phytoplankton, ideally pulsed feeding to overwhelm the polyps.
NPS gorgonians are some of the hardest livestock in the hobby. Don't keep them unless you're committed to twice-daily feeding indefinitely.
The dosing habit
For tanks with multiple filter feeders, build a dosing routine:
- Live phyto every day or every other day, after lights-out
- Skimmer and UV off for 30 to 60 minutes around dosing
- Stable nutrient levels (avoid ULNS)
- Watch the animals. Open clams, fully-extended feather dusters, and active polyps tell you the dosing rate is right.
The bottom line
Filter feeders aren't difficult, they're just dependent on consistent feeding. Live phytoplankton, stable nutrients, and a refugium that produces continuous microfauna will keep most of them thriving.
If you can't commit to regular phyto dosing, skip the picky filter feeders and stick to corals and macroalgae. They'll forgive a missed week. Sponges and sea squirts won't.
