What matters in a planted-tank filter
Three variables dominate filter selection for a planted tank:
1. Tank size. Flow rate should turn over the tank volume 4–6× per hour for planted setups. Less and you get dead spots where detritus accumulates; more and you blow plants around. “Rated” flow on filter packaging is measured without media; actual flow is typically 40–60% of rated. A 20-gallon tank wants a filter rated 150–250 GPH to deliver 100–150 GPH of actual flow.
2. Livestock. Shrimp and fry need sponge filters or pre-filter sponges over intakes. Adult fish don’t care about intake type. Schooling nano fish appreciate gentle flow; territorial fish like cichlids tolerate stronger currents.
3. Planting density. Heavily planted tanks do significant biological filtration through the plants themselves, since they consume ammonia as a nitrogen source. This reduces the burden on filter biological media. Sparsely planted tanks rely more heavily on filter bacteria for ammonia processing.
The three filter types
Canister filters (Fluval 07 series, Oase BioMaster, Eheim Pro) sit in the cabinet below the tank with input and output hoses. Quietest option, most media capacity, best for aesthetics (only two hoses visible in-tank). Cost 3× an HOB but justify it for large tanks or aesthetics-focused setups. Best for: 29-gallon+, aquascaped tanks, keepers who want silence.
HOB (hang-on-back) filters (AquaClear, Aqueon Quietflow, Tidal) clip on the back edge of the tank with an intake tube and a waterfall output. Cheapest reliable option, easiest to maintain (2-minute media rinse), visible on the back of the tank. Best for: 10–40 gallon tanks, budget builds, keepers who want simplicity.
Sponge filters are sponges with an air pump driving water through them via air uplift. Cheapest option, safest for shrimp and fry, visible in-tank, lowest maintenance. Biological filtration only, with no mechanical clarity to speak of. Best for: shrimp tanks, fry grow-out tanks, hospital tanks, backup filtration.
What “good” looks like across types
Regardless of filter type, a good filter for a planted tank has:
- Adjustable flow to throttle down for newly-planted tanks and up as plants fill in
- Generous biological media volume for stable bacterial colonies
- Easy-access maintenance. If you can’t rinse the media in under 5 minutes, the filter gets neglected
- Reliable parts availability. Impellers and O-rings wear out; if replacements are hard to find, the filter becomes disposable
Common filter mistakes
- Oversizing the filter and running it at full flow (blasts plants)
- Undersizing because “plants do the filtering” (they do some, not all)
- Using proprietary cartridges when generic media works better
- Never cleaning the mechanical sponge (clogs and bypasses)
- Cleaning biological media in tap water (kills beneficial bacteria)