Aquarium Filtration at a Glance
Every aquarium needs three types of filtration: mechanical (removes particles), biological (converts toxins), and chemical (removes impurities).
Filter Type Comparison by Need
Filter Types Compared
| Filter Type | Best For | Media Capacity | Flow | Cost | Maintenance |
|---|---|---|---|---|---|
| Hang-on-Back (HOB) | Small to medium tanks (5-55 gal) | Limited | Good | $20-100 | Easy (monthly) |
| Canister | Medium to large tanks (30-200+ gal) | Large | Excellent | $80-400 | Moderate (quarterly) |
| Sponge | Breeding, fry, shrimp tanks | Minimal | Gentle | $5-30 | Easy (squeeze) |
| Sump | Large/reef tanks (75+ gal) | Maximum | Excellent | $200-500+ | Moderate (monthly) |
| Internal | Small tanks, quarantine | Limited | Moderate | $10-50 | Easy (monthly) |
Filtration System Flow
Do's and Don'ts
| Choose a filter rated for 2x your tank volume (over-filtering is better than under) | |
| Use mechanical (sponge/floss) + biological (ceramic rings/bioballs) media at minimum | |
| Clean filter media in old tank water — NEVER under tap water (kills bacteria) | |
| Add chemical media (carbon, purigen) when needed — not continuously | |
| Ensure water flows through ALL media types — don't let one bypass another | |
| Replace all biological media at once — you crash the nitrogen cycle | |
| Run a filter without any biological media — bacteria are the real filter | |
| Use underpowered filters — insufficient flow causes dead spots and poor filtration | |
| Clean filter sponges until they are 'squeaky clean' — some bacteria should remain |
💡 Your filter does not clean the water — bacteria in the filter clean the water. The mechanical media traps particles, but the biological media growing beneficial bacteria is what converts toxic ammonia to safe nitrate. Protect your bacteria colony.
⚠️ Most beginner tanks fail because the filter is too small or cleaned improperly. A HOB filter rated for a 20-gallon tank on a 20-gallon tank is the minimum, not the ideal. Always buy a filter rated for a larger tank than you have.
Aquarium filters perform three types of filtration: mechanical (traps particles), biological (bacteria convert ammonia to nitrate), and chemical (activated carbon removes dissolved compounds). Biological filtration is the most important. The filter media that hosts beneficial bacteria must never be cleaned with tap water or replaced all at once. For most home tanks, hang-on-back filters are the best combination of performance and ease. Canister filters are better for larger or heavily stocked tanks.
The Three Filtration Types
Clean: Rinse in tank water (not tap) when flow drops.
Never: Clean with tap water (chlorine kills bacteria) or replace all at once.
Replace: Every 2–4 weeks. Remove carbon before medicating the tank.
Filter Types: Which Should You Choose?
| Filter Type | Best Tank Size | Cost | Maintenance | Pros | Cons |
|---|---|---|---|---|---|
| Hang-on-Back (HOB) | 10–75 gal | $20–$80 | Low | Easy access, good surface agitation, reliable, widely available | Noisy, evaporation, less media capacity than canister |
| Canister Filter | 30–250+ gal | $60–$300 | Moderate | Large media capacity, quiet, flexible media choices, hidden under tank | Higher upfront cost, less surface agitation, messier to clean |
| Sponge Filter | 5–30 gal | $5–25 | Very low | Cheap, safe for fry and shrimp, excellent bio-filtration, easy to clean | Requires air pump, weaker mechanical filtration, visible in tank |
| Internal Power Filter | 5–40 gal | $15–50 | Low | Fully submerged, compact, quiet, good for small/quarantine tanks | Takes up tank space, limited media, less powerful than HOB/canister |
| Undergravel Filter (UGF) | 10–55 gal | $10–30 | High | Inexpensive, no visible equipment, uses substrate as bio-media | Traps detritus under gravel, incompatible with planted tanks, outdated |
| Wet/Dry Sump | 75+ gal | $150–$500+ | Moderate | Maximum bio-filtration, large water volume, hides equipment | Complex setup, requires drilled tank or overflow box, costly |
Mechanical Filtration
Physically traps suspended particles in foam, sponge, floss, or felt media. The trapped waste must be periodically removed by rinsing or replacing the mechanical media. If mechanical media becomes clogged and is not cleaned, the trapped waste decomposes in the filter and releases ammonia directly back into the water.
Rinse mechanical media in a bucket of tank water removed during a water change. Never under tap water — chlorine kills the beneficial bacteria living in it.
Biological Filtration
The most critical filtration type. Beneficial bacteria (primarily Nitrosomonas and Nitrospira genera) colonize porous surfaces in the filter and convert toxic ammonia to nitrite (still toxic), then nitrite to nitrate (tolerable at low levels). Without established biological filtration, ammonia accumulates and kills fish.
Biological media: sponge foam, bio-balls, ceramic rings, lava rock, and specialized media like Seachem Matrix. Ceramic rings and Matrix have the most surface area per volume. Never replace all biological media at once — do no more than half, and only when flow is significantly reduced.
Chemical Filtration
Activated carbon is the most common chemical media. It adsorbs dissolved organics, chlorine, certain medications, tannins from driftwood, and odors. It does not remove ammonia, nitrite, or nitrate. Activated carbon becomes saturated and ineffective after 2–4 weeks and must be replaced. Critical: remove carbon before treating the tank with any medication — carbon will absorb the medication, rendering it ineffective.
Filter Maintenance Schedule
- Rinse mechanical media in tank water
- Check impeller for debris
- Replace activated carbon
- Test flow rate (reduced flow = clogged media)
- Clean filter housing
- Inspect O-rings and seals (canisters)
- Consider replacing sponge if deteriorating
- Never replace all bio-media at once
- Rinse filter media under tap water
- Replace all bio-media at once
- Turn off filter for more than 30 min (bacteria starve)
- Use bleach to clean filter parts
How to Size Your Filter
Filter manufacturers rate their products in gallons per hour (GPH). The traditional guideline is 4–6x the tank volume per hour. A 30-gallon tank needs a filter rated for 120–180 GPH minimum.
Adjust upward for:
- High bioload fish (goldfish, oscars, heavily stocked community tanks): use 8–10x the tank volume
- Heavily planted tanks: can use lower turnover since plants assist with biological filtration
- Messy species (goldfish, cichlids): double the baseline recommendation
Running two smaller filters rather than one large filter provides redundancy — if one fails, the other maintains biological filtration while you troubleshoot.