Why HOB Filters Are the Default Choice
I've run AquaClear 20s and 50s for four years without a single failure. They are the filters I recommend to every new aquarist setting up a tank between 10 and 55 gallons, and the ones I default to myself on any tank that does not specifically need a canister. They strike the best balance of price, performance, and ease of maintenance of any filter category.
The HOB design is simple: a pump lifts water out of the tank through an intake tube, pushes it through a media chamber, and returns it over a waterfall spillway. The media chamber is accessible from the top without disconnecting anything, which makes maintenance a five-minute job. The pump is integrated into the body, so there is no separate air pump to buy. And the entire unit hangs on the back of the tank, so it does not eat swimming space.
The trade-off is the waterfall return, which creates surface agitation. For most community tanks that is a feature — it oxygenates the water. For high-tech planted tanks running CO₂, it is a bug, because it strips CO₂ out as fast as you can inject it. For most other uses, HOBs are the right call.
Size your HOB to turn over the tank volume 4 to 6 times per hour. A 20-gallon tank needs a HOB rated for at least 80 to 120 GPH. For messy fish (goldfish, large cichlids), aim for 8 to 10 times per hour. Always buy a filter rated for a larger tank than yours — manufacturer ratings are optimistic.
Sizes of HOB Filters
HOB filters come in three practical size ranges. Pick the one that matches your tank.
Small HOB Filters (10 to 30 gallon tanks)
Small HOBs are rated for 10 to 30 gallons and flow at 80 to 150 GPH. They are compact, quiet, and cheap. The AquaClear 20 and 30 are the gold standards here, but there are dozens of competitors. Small HOBs work well on betta community tanks, 10-gallon quarantine setups, and 20-gallon longs. Avoid the cheapest no-name units — their impellers fail within a year.
Best for: 10 to 30 gallon community, quarantine, and betta tanks.
Medium HOB Filters (30 to 55 gallon tanks)
Medium HOBs cover 30 to 55 gallons at 150 to 300 GPH. They are the most common HOB size in the hobby. The AquaClear 50 and 70 sit in this range and are reliable workhorses. Medium HOBs have a media chamber large enough to run a proper stack of sponge, ceramic, and carbon. They are the right choice for most community tanks.
Best for: 30 to 55 gallon community tanks, planted low-tech tanks, grow-out tanks.
Large HOB Filters (55 to 75 gallon tanks)
Large HOBs like the AquaClear 110 push 300 to 500 GPH and cover 55 to 75 gallon tanks. They are physically large — check that your tank stand has the headroom for one. A large HOB is a reasonable alternative to a canister for a 55 or 75 gallon tank if you want simplicity over capacity.
Best for: 55 to 75 gallon community tanks, goldfish tanks, and tanks where a canister is too expensive or complex.
How to Choose the Right HOB Filter
Sizing
| Tank Size | HOB Size | Min Flow Rate | Media Chamber |
|---|---|---|---|
| 10–20 gal | Small | 80–150 GPH | Single cartridge |
| 20–40 gal | Small/Medium | 150–200 GPH | Single chamber |
| 40–55 gal | Medium | 200–300 GPH | Stacked media |
| 55–75 gal | Large | 300–500 GPH | Stacked media + extension |
For tanks over 75 gallons, a single HOB cannot keep up. Either run two large HOBs (one at each end of the tank for flow distribution) or step up to a canister filter.
Features to Look For
- Open media chamber: Filters with a single replaceable cartridge are cheap to run for the first year and expensive forever after. Look for filters with an open chamber you can fill with your own sponge, ceramic, and carbon.
- Adjustable flow: A flow-control dial lets you tune output for betta and fry tanks. Essential for any tank with weak-swimming fish.
- Surface skimmer: Some HOBs include a skimmer attachment that pulls surface film off the top of the water. Useful for tanks with oily surface film from food.
- Easy priming: Most HOBs self-prime after the first fill, but some need a manual prime each time the power goes out. Self-priming is worth the extra money.
- Pre-filter sponge on intake: A sponge over the intake prevents small fish, shrimp, and fry from being sucked in. Buy one aftermarket if the filter does not include it.
- Quiet motor: Impeller design matters. Look for filters with a rubber-mounted impeller well — they are noticeably quieter.
Most cheap HOBs ship with a single-use cartridge and a recommendation to replace it monthly. Do not. The cartridge is where 90% of your beneficial bacteria live — replacing it causes a mini-cycle that can spike ammonia and kill fish. Rinse the cartridge in old tank water and only replace it when it is physically falling apart. Better yet, swap the cartridge for a permanent sponge.
Recommended HOB Filters
Placeholder picks across the three tiers I recommend. I will add specific model links and pricing as I test them.
[Small HOB 100 GPH]
Best for: 10 to 20 gallon community tanks. Replaceable cartridge (swap for sponge), adjustable flow, quiet operation. The reliable starter filter.
[Medium HOB 200 GPH, Open Chamber]
Best for: 30 to 55 gallon community and planted tanks. Open media chamber, stacked sponge/ceramic/carbon, adjustable flow. The HOB most aquarists should buy.
[Large HOB 500 GPH with Skimmer]
Best for: 55 to 75 gallon community and goldfish tanks. Built-in surface skimmer, large media capacity, near-silent operation. The HOB alternative to a canister.
Common HOB Filter Mistakes
- Replacing the cartridge monthly. The single biggest beginner mistake in the hobby. Rinse, do not replace.
- Cleaning with tap water. Chlorine kills the bacteria. Rinse media in old tank water only.
- Letting the water level drop. A low water level turns the waterfall into a noisy gurgle and adds oxygen disruption. Keep the tank full.
- No pre-filter sponge. Without one, fry, shrimp, and small fish get sucked into the intake. A $2 sponge prevents this.
- Running the filter dry. If the water level drops below the intake, the pump runs dry and burns out in minutes.
- Ignoring the impeller. The impeller well collects calcium and biofilm. Clean it every 2–3 months with a small brush.
- Replacing all media at once. When you finally do swap the cartridge, run the new one alongside the old for two weeks to seed it.
Frequently Asked Questions
What size HOB filter do I need?
Size your HOB filter to turn over your tank volume 4 to 6 times per hour. A 20-gallon tank needs a HOB rated for at least 80 to 120 GPH. For messy fish like goldfish or heavy stocking, aim for 8 to 10 times per hour. Always check the actual GPH rating, not just the tank size on the box.
How often should I replace the cartridge in a HOB filter?
Never replace the entire cartridge at once. The cartridge is where most of your beneficial bacteria live — throwing it away causes a mini-cycle that can spike ammonia. Rinse the cartridge in old tank water monthly and only replace it when it is physically falling apart, then run the old cartridge alongside the new one for two weeks to seed it.
Are HOB filters noisy?
HOB filters can be noisy if the water level drops too low (causing a gurgling waterfall sound) or if the impeller gets clogged with debris. A properly maintained HOB with the tank filled to the correct level is quiet. If yours is loud, check the water level, clean the impeller well, and replace the impeller shaft if it is worn.
Can a HOB filter run a planted tank?
Yes, but with a caveat. The waterfall return of a HOB filter creates surface agitation that drives off CO2, which planted tanks often supplement. For low-tech planted tanks without CO2 injection, a HOB is fine. For high-tech tanks with CO2 injection, a canister filter is preferable because it does not strip CO2 from the water.
Recommended Products
No brand bias. These are product categories we recommend based on real fishroom experience. Affiliate links may be added in the future.
Sponge Filter + Air Pump
Best for: Shrimp tanks, fry tanks, and low-budget setups.
Cheap, nearly indestructible, shrimp-safe, biological filtration only.
HOB Filter (Aquaclear 20)
Best for: 10–20 gallon community tanks.
Mechanical + biological + chemical, easy to maintain, reliable.
Canister Filter (Fluval 107)
Best for: 20–40 gallon planted or heavily stocked tanks.
Silent, high media capacity, superior water clarity, low maintenance.