Choosing a Canister Filter for a Planted Tank

Introduction

Most people buy a canister filter to keep the water clear. In a planted tank that is the smaller half of the job. The bigger half is moving water — carrying CO2-rich, fertilised water through every stem, every leaf and every inch of carpet, evenly enough that no corner of the layout sits still.

That changes how you size a filter. A canister rated "up to 66 gallons" is rated for a lightly stocked community tank, not a densely planted scape running pressurised CO2. Here is how we size canisters for planted tanks, using the UNS Delta range as the reference.

Why Planted Tanks Need Flow, Not Just Filtration

In a fish-only tank the filter holds biomedia and strips out waste; water movement is incidental. In a planted tank, water movement is the delivery system for everything the plants need.

• CO2 distribution — dissolved CO2 does not spread evenly through still water. If your drop checker is lime green at the front left and blue at the back right, that is a flow problem, not a dosing problem.
• Fertiliser delivery — water column ferts only work where the water actually goes.
• Boundary layer — every leaf carries a thin film of static water at its surface. Gentle constant current strips it away so the leaf can exchange gas and take up nutrients.
• Detritus transport — flow lifts fine mulm off the substrate and carries it to the intake instead of letting it settle in the carpet, where it feeds algae.

This is why aquascapers choose canisters over hang-on-back filters for anything beyond a nano: you can put the intake at one end and the return at the other, setting up a deliberate circuit across the whole layout rather than a churn in one corner.

The 5–10x Turnover Rule, and Where It Misleads You

The rule of thumb is 5 to 10 times the tank's volume through the filter every hour — a 20-gallon tank wants roughly 100–200 gallons per hour. Low-tech setups sit at the bottom of that band; high-light, CO2-injected, densely carpeted scapes sit at the top.

The catch is that the headline flow figure on almost any canister is measured with an empty body and no hose — the pump running against nothing. Add media, hose length, lift to the rim and an inline diffuser, and real delivery drops substantially. UNS is unusually straight about this: every Delta publishes both a max flow rate and a lower circulation flow rate. Use the circulation figure for your turnover maths, and leave headroom on top: as the sponges load with detritus, output falls noticeably by week four.

Sizing a Delta to Your Tank

The table pairs each Delta against the tank volume we would actually put it on for a planted layout. That is deliberately more conservative than the rated capacity — the gap is the difference between keeping a volume of water clean and turning it over enough times per hour to run plants properly.

The two smaller sizes are also sold on their own as the current revision — the Delta 30 V2 and the Delta 60 V2.

When a Delta Is Not the Right Filter

• Over about 45 gallons. On a 90P or a 120P a single Delta 120 will keep the water clean but will not give you 5x turnover. Run two canisters at opposite ends, or step outside the range.
• Pico tanks under about 3 gallons. A canister is overkill and the intake is a hazard. A sponge or corner filter is the better answer.
• Tanks you move often. Canisters mean plumbing. If the setup is temporary, a hang-on-back is less hassle.

Dead Spots Are Where Algae Starts

A dead spot is any area where water is effectively still. You can find them in thirty seconds: turn the CO2 off, drop a pinch of fine food into the water column and watch where it settles rather than travels.

Two things go wrong there. Organic waste accumulates and decomposes. And the plants in that zone get less CO2 and fewer nutrients, so they grow weakly. Weak plants plus high organics is the standard recipe for black beard algae and cyanobacteria — which is why those two reliably show up in back corners and at the base of dense hardscape.

Fixing Flow Without a Bigger Filter

• Aim the return along the front glass, not across the middle. The current runs the length of the tank, hits the far wall and returns along the back — a full circuit instead of a jet through the centre.
• Put intake and outflow at opposite ends so water has to cross the layout to get back.
• Use lily pipes. A wide flared outflow like the Aquario Neo Flow set spreads the return into a broad soft current instead of a narrow jet. The same flow rate does far more work.
• Trim for flow, and clear the surface film — oily biofilm blocks gas exchange, and a surface skimmer removes it permanently.

Media Order Inside the Canister

Water travels bottom to top through stacked baskets. The principle is coarse first, fine last, so the cheap media protects the expensive media.

1. Coarse sponge. Catches leaf fragments and trimmings; the layer you rinse most often.
2. Biomedia — the largest share of the space. Ceramic rings, sintered glass or bio-balls, where your nitrifying bacteria live. On a three-basket Delta, two baskets of biomedia is a sensible split.
3. Fine floss or wool. Polishes the water, clogs fastest, gets rinsed or replaced at every service.

Two things to leave out. Carbon has a short useful life and strips the chelated traces you are paying to dose — keep it for pulling medication out after a treatment, then remove it. Phosphate removers are counterproductive; phosphate is a plant nutrient you dose on purpose. Rinse sponges in old tank water, and stagger biomedia replacement rather than swapping it all at once.

Running an Inline CO2 Diffuser on the Return

Once you have a canister, the return line is the best place to inject CO2. An inline atomizer sits in the hose between canister and tank, and the pump forces CO2 into a mist fine enough to dissolve almost completely before it reaches the outflow. Nothing sits in the tank, distribution is even because the gas enters where the current starts, and there is no ceramic disc to scrub.

Match the atomizer to your tubing diameter, not to your tank: 12/16mm plumbing takes the Qanvee M1, 16/22mm takes the M2. Fit it on the return side, after the canister — never on the intake, where gas collecting inside the canister body will make the pump lose prime and run dry.

Two caveats. Inline atomizers create back pressure, so you will usually run the regulator a little higher than with a glass diffuser. And the fine bubbles can leave a faint mist in the water column during the photoperiod — most people stop noticing within a week, some never make peace with it. While you are plumbing, soft matte grey silicone tubing hides against a wall far better than the stiff green hose most filters ship with.

Frequently Asked Questions

Can a canister filter be too powerful for my tank?

Rarely in terms of filtration — more biomedia is never a problem. But you can create too much current for fine-leaved plants and for fish like bettas. Do not throttle the pump; spread the flow instead, with a wider outflow or by aiming the return along the front glass.

How often should I clean a planted tank canister?

Every six to eight weeks for the mechanical stages is a reasonable start, adjusted by how fast flow visibly drops. Do not clean it in the same week as a big trim and a large water change.

Should the CO2 go before or after the filter?

After. Always plumb an inline atomizer into the return line, downstream of the canister. On the intake it sends gas into the canister body, where it pockets at the top and starves the impeller.

Is one big canister better than two small ones?

On tanks over roughly 40 gallons, two smaller canisters at opposite ends usually beat one large one: flow from two directions, redundancy if a pump fails, and you can service one while the other keeps the biology running.