# Terrestrial Plants as Aquarium Filters: Building a Plant-Based Filtration System I spent my first six months in this hobby throwing money at every bit of kit the aquatics shop suggested, convinced that more equipment would solve my water quality problems. Expensive canister filters, UV sterilisers, chemical media that needed replacing every month. My nitrates still sat stubbornly at 40-50 ppm despite weekly water changes, and I was starting to wonder if I’d bitten off more than I could handle.
Then I watched a YouTube video about someone growing pothos cuttings in their sump and decided to give it a go. Cost me nothing except a few minutes with a pair of scissors and my wife’s houseplant. Three weeks later, my nitrates had dropped to 15 ppm and stayed there.
That was two years ago, and I haven’t looked back. If you’re reading this as someone who’s frustrated with conventional filtration or just curious about whether plants can actually do the work of expensive equipment, the answer is yes.
Not completely, but more than you might think. ## The Science Behind Plant-Based Filtration Plant-based aquarium filtration works because terrestrial plants growing in water actively consume dissolved nutrients that would otherwise accumulate as waste products in your tank.
The process is straightforward: your fish produce ammonia through respiration and waste, beneficial bacteria convert that ammonia to nitrite and then nitrate, and the plants absorb the nitrate as a nitrogen source for growth. **Nitrate Absorption Rates**: Research shows significant differences between plant types. Emergent tropical arums like pothos and peace lily demonstrate about 75% nitrate removal potential compared to surface water plants (Aquarium Science). Annual emergent plants like vegetables achieve roughly 50% nitrate removal potential versus surface water plants (Aquarium Science).
What this means practically is that a pothos cutting will pull more nitrate from your tank water than most floating aquatic plants, and considerably more than vegetables like basil or lettuce. **Documented Performance**: Real world results back up the theory. Pothos shows nitrate reduction ranges of 20-40 ppm in established systems (Plant Index), with noticeable changes occurring within 4-5 days (Foliage Friend). One documented case showed nitrates dropping from 80 ppm to 40 ppm in five days after adding four pothos cuttings to the system (BFE Fish Tank), with levels stabilising at 10-15 ppm after two months when combined with smaller 20% weekly water changes rather than the previous larger changes (BFE Fish Tank).
The plants work continuously, not just during feeding times or when you remember to dose liquid fertiliser. As long as there are nitrates present and the plants are healthy, they’re pulling nutrients from the water column twenty-four hours a day. ## Primary Implementation: Hang-On-Back Plant Filtration The most practical approach for most people is setting up a hang-on-back plant filter system. This works with any tank where you can position a container above the water level, allowing water to flow through plant roots and back into the tank. **Basic Setup Requirements**: You need a small pump capable of lifting water 30-40cm (depending on your tank rim height), airline tubing, a container to hold the plants, and some form of growing medium.
I use a basic internal filter pump rated at 200 litres per hour, which costs about £15-20. The container can be anything from a repurposed takeaway box to a purpose-built acrylic trough. Growing medium options include expanded clay pebbles, rockwool, or even just water if you’re growing cuttings. **Plant Selection and Positioning**: Pothos is the obvious first choice because it’s nearly impossible to kill and grows rapidly in aquarium conditions.
Peace lily works well if you want something that flowers. Both tolerate varying light conditions and don’t mind having their roots constantly wet. Position plants so roots are submerged but leaves stay above water level.
Root systems need access to dissolved oxygen, which means either moderate water flow or periodic exposure to air. **Flow Rate Considerations**: You want enough flow to keep roots oxygenated without creating a waterfall effect that splashes everywhere or stresses fish with noise. About 2-3 times tank volume per hour through the plant section works well for most setups. A 60-litre tank would use a 120-180 litres per hour pump.
Too little flow and roots may rot from lack of oxygen. Too much and you’re just moving water around without giving plants time to absorb nutrients. **Maintenance Requirements**: Cheque water levels weekly as evaporation is higher with exposed growing surfaces. Trim plant growth monthly or whenever it starts looking overgrown.
Roots may need occasional cleaning if they collect debris, but this usually happens naturally with water flow. Replace growing medium every 6-12 months or when it starts breaking down, though clay pebbles last years if rinsed occasionally. **UK Specific Considerations**: Winter heating costs matter if you’re running this in an unheated room. Plants slow their growth in cold conditions, reducing filtration efficiency.
Summer heat waves can stress plants if your tank room gets above 28°C regularly. Most houseplants used for aquarium filtration prefer consistent temperatures between 18-25°C, which matches what your tropical fish need anyway. ## Common Mistakes to Avoid
Mistake #1: Using tap water for top-offs without dechlorinating. Municipal water treatment includes chlorine for disinfection (US EPA), which kills beneficial bacteria and can damage plant roots. Always dechlorinate replacement water, just as you would for fish.
This becomes more important with plant systems because you’re topping off more frequently due to increased evaporation.
Mistake #2: Expecting plants to eliminate the need for water changes entirely. Plants reduce nitrate accumulation but don’t remove dissolved organics, replenish trace elements, or maintain pH stability. You still need regular water changes, just smaller or less frequent ones. The goal is reducing maintenance burden, not eliminating it completely.
Mistake #3: Overcrowding the plant growing area. Roots need space and water flow to function properly.
Cramming too many plants into a small container leads to stagnant water, root rot, and reduced filtration efficiency. Start with fewer plants and add more gradually as you see how they perform in your specific setup.
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Mistake #4: Ignoring light requirements above water. Plants growing emersed need adequate light for photosynthesis just like any houseplant. A standard aquarium light won’t provide enough illumination for leaves growing 20-30cm above the tank.
You may need additional lighting or position the system near a window with indirect sunlight.
Mistake #5: Using plants that can’t handle constant root submersion. Not all houseplants tolerate having their roots permanently underwater. Succulents, most herbs, and plants adapted to dry conditions will rot quickly. Stick to plants specifically known to thrive in water culture or hydroponic systems.
## Research Support Multiple studies demonstrate the effectiveness of terrestrial plants in aquatic filtration systems.
The documented nitrate removal rates showing 75% efficiency for tropical arums versus traditional aquatic plants provide a clear advantage for anyone dealing with elevated nitrate levels (Aquarium Science). The practical reports of nitrate reductions from 80 ppm to stabilised levels of 10-15 ppm within two months demonstrate real-world applicability rather than just theoretical possibility (BFE Fish Tank). With over 25 documented houseplant species capable of growing in water culture (Real Simple), hobbyists have numerous options beyond the commonly discussed pothos and peace lily combinations. ## Application to Different Tank Systems **Nano Tanks (20-40 litres):** Use single pothos cuttings in small containers positioned above the tank.
Simple airline tubing can create adequate flow without overpowering small pumps. Monitor closely as plant growth may outpace nutrient production in lightly stocked nano setups. **Community Tanks (60-200 litres):** Multiple plant species in larger growing containers work well. Consider peace lily, philodendron, and pothos combinations for varied root structures and growth patterns.
Flow rates of 200-400 litres per hour through the plant section suit most community tank bioloads. **Heavily Stocked Tanks:** Plant filtration works as supplemental nitrate removal but cannot replace adequate mechanical and biological filtration. Use larger plant growing areas or multiple smaller units. Monitor nitrate levels more frequently during the establishment period. **Cichlid Systems:** Robust plants like pothos and peace lily handle the higher bioload typical of cichlid tanks.
The additional nitrate removal helps manage the waste output from large, messy fish without increasing water change frequency dramatically. **Saltwater Applications:** Limited research exists on salt tolerance for most terrestrial plants used in freshwater plant filtration. Stick to conventional filtration methods for marine systems until more definitive guidance becomes available. ## Benefits of Plant-Based Filtration **Reduced Water Change Frequency**: With nitrates maintained at 10-20 ppm instead of climbing to 40-50 ppm between changes, you can extend water change intervals from weekly to fortnightly for many tanks, saving time and water treatment costs over the year. **Lower Long-term Equipment Costs**: Plants don’t need replacement media, motor servicing, or electrical components that eventually fail. Initial setup costs under £50 for most systems, with ongoing expenses limited to occasional plant replacement and growing medium refresh every year or two. **Natural Waste Processing**: Unlike mechanical filtration that removes waste particles or chemical media that absorbs dissolved compounds, plants actually consume waste nutrients and convert them to plant tissue.
This removes waste from the system entirely rather than just collecting it for later removal. **Improved Water Stability**: Continuous nutrient uptake helps prevent the gradual accumulation of dissolved organics that can lead to pH swings and water chemistry problems. Plants buffer water chemistry changes more gradually than equipment-based systems. **Educational Value**: Watching plants grow while maintaining your tank provides direct feedback about water quality and system health. Rapid plant growth indicates adequate nutrients, while slow growth or yellowing leaves suggest problems worth investigating. **Aesthetic Integration**: Well-designed plant filtration systems become attractive features rather than equipment you need to hide.
Growing herbs above your tank gives you fresh basil for cooking while cleaning your aquarium water. ## Step-by-Step Implementation Guide **Step 1: Assessment and Planning (Week 1)** Measure current nitrate levels and determine tank rim accessibility for hang-on equipment. Order basic components: 200 L/h internal pump (£15-20), airline tubing (£3), growing container (£5-10), expanded clay pebbles (£8-12). Take cuttings from existing houseplants or purchase pothos plants (£3-8 each).
Test cuttings in glasses of dechlorinated water to ensure they root properly before committing to the system. **Budget Breakdown:** * Pump and tubing: £18-25 * Container and growing medium: £13-22 * Plants: £3-15 * **Total Budget: £35-60** **Step 2: Basic System Installation (Week 2-3)** Install pump and container, establish water flow through growing medium. Add rooted plant cuttings with roots fully submerged and leaves above water level. Monitor for leaks, adequate flow rate, and pump noise levels.
Adjust flow if necessary and ensure system operates reliably for 7-10 days before expecting filtration benefits. **Step 3: Performance Monitoring (Week 4-8)** Test nitrate levels weekly to track effectiveness. Expect initial reduction within 4-7 days, with stabilisation occurring over 4-6 weeks.
Adjust water change schedule gradually based on nitrate trends.
Document plant growth rates and root development to identify which species perform best in your specific water conditions. **Step 4: System Optimisation (Month 3-4)** Add additional plant species or increase growing area based on initial performance results. Fine-tune flow rates and lighting if plant growth seems slow. Establish maintenance routine for trimming, cleaning, and replacing growing medium.
Calculate long-term cost savings from reduced water change frequency and chemical media usage. **Budget for Full System:** * Enhanced container system: £25-40 * Additional plants and media: £15-25 * Lighting supplements: £20-35 * **Total Comprehensive Budget: £95-160** **Step 5: Long-term Management (Month 4+)** Integrate plant maintenance into regular tank care routine. Harvest excess plant growth for new setups or sharing with other hobbyists. Monitor seasonal variations in plant performance and adjust expectations during winter months when growth naturally slows.
Track annual savings in water treatment costs and equipment replacement compared to conventional filtration approaches. The beauty of plant-based filtration is that it starts simple and scales with your interest and results. My first setup was a pothos cutting in a margarine tub powered by a £12 pump.
Two years later I’m growing herbs above three different tanks and wondering why it took me so long to figure this out. Sometimes the best solutions are the ones that have been hiding in plain sight all along.
