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The answer first
Measure the outside diameter of your filter’s intake tube and buy the AQUANEAT sponge in that size. Nothing else about the product matters. It is sold in 0.4, 0.6, 0.8 and 1 inch, six to a pack for about six dollars, and the size is printed in the listing name - which is more than most aquarium products manage about the one dimension that decides whether they work.
0.8 inch fits most nano hang-on-backs. 0.6 fits the slimmer internal filters. Guessing between them is the only way to waste the purchase: a loose sponge slides off within a week and does nothing, and one that has to be stretched on splits at the seam.
If you are buying a filter rather than modifying one, consider a sponge filter instead and skip the problem. A sponge filter has no impeller to guard, which is why it is this site’s default for a 5-gallon. A pre-filter sponge on a sponge filter is a maintenance job with no benefit.
Why this page exists
We recommend fitting one of these in fourteen places across this site - including a callout on the 5-gallon filter page saying a pre-filter sponge is not optional at that size - and until now we never said which one to buy or how to size it. That is the gap this page closes. It is a two-dollar component, and it does three useful things at once.
What it actually does - three jobs, one object
- 01
It keeps fins out of the impeller
A hang-on-back intake is a slot with suction behind it. A long-finned betta that drifts past gets a trailing fin pulled against the strainer, and on a bad day into it. A foam cover means the suction is spread across the whole surface of the sponge instead of concentrated at a slot.
- 02
It keeps shrimp and snails out of the impeller
This is the version of the job most people buy it for. An adult amano is usually too big to enter an intake; a shrimplet is not, and neither is a small ramshorn. Our shrimp pages recommend this on every one of them.
- 03
It reduces flow a little, which in a nano tank is usually welcome
A foam cover is a restriction. On a filter already pushing more turnover than a betta wants, that is a side benefit rather than a cost - though it is not a substitute for a proper baffle, which is a different page.
- 04
It adds a little mechanical and biological filtration
The foam traps debris before it reaches the cartridge, and colonises with bacteria like any other media. Neither effect is large in a nano tank. Both are free once the sponge is there for the first two reasons.
How to measure, in one minute
The mistake is measuring the filter, or trusting the filter’s model number. What you need is the outside diameter of the intake tube - the plastic pipe that runs down into the water.
- Unplug the filter first. An impeller that starts while your hand is in the intake is an unpleasant surprise rather than an injury, but unplug it anyway.
- Lift the intake out and measure across the tube with calipers if you have them, or against a ruler if you do not. You are measuring the widest part of the round tube, not the strainer basket at the bottom.
- Round to the nearest size AQUANEAT sells - 0.4, 0.6, 0.8 or 1 inch. The foam stretches a little, so nearest is fine; two sizes out is not.
- If the strainer basket is wider than the tube, as it is on several nano filters, you have a choice: a sponge sized to the tube that sits above the basket, or a larger one pushed down over the basket itself. The second covers more and is the one we would do.
For reference, the filters this site recommends run intakes in the 0.6 to 0.8 inch range, which is why those are the two sizes picked here. At six dollars a pack, buying both and keeping the spare is a defensible way to avoid measuring twice.
What is published, and what is not
| Specification | Published? | Why it matters |
|---|
| Intake diameter | Yes - in the listing name | The only dimension that decides fit |
| Quantity per pack | Yes - 6, also sold as 12 | These are consumables, not a one-time purchase |
| Intended use | Yes - shrimp and fry protection | Confirms the maker means it as an intake guard |
| Pore size (PPI) | No | Decides how fast it clogs and how much flow it costs. Nobody in this category publishes it |
| Flow restriction | No | A clogged pre-filter is the commonest cause of a filter that was quiet becoming loud |
| Manufacturer spec page | No - AQUANEAT serves none | Everything here is read off the retail listing and labeled as such |
The pore-size absence is the one with a practical consequence. Without a PPI figure there is no way to predict how often a given sponge needs rinsing, so the honest instruction is behavioral rather than numerical: rinse it when you do the water change, in the bucket of old tank water rather than under the tap, and rinse it sooner if the filter output drops or the motor note rises.
This is not a baffle
A pre-filter sponge restricts flow a little as a side effect. It does not redirect the outflow, which is the thing that actually pushes a betta around a small tank. Baffling the outflow is a separate ten-minute job on the other end of the filter, and a tank with a betta fighting the current needs both.
Maintenance, and the one mistake that matters
A pre-filter sponge is a consumable. It discolors within weeks, which is cosmetic, and eventually the foam perishes and tears, which is not. Six to a pack exists for that reason.
The mistake worth naming: never rinse it under a tap. The sponge colonises with the same nitrifying bacteria as the rest of your media, and in a nano tank that colony is not large enough to lose casually. Chlorinated tap water kills it. Squeeze the sponge out in the bucket of water you have just removed from the tank, put it back wet, and the colony survives the cleaning.
The same logic applies to replacing it. Swapping a matured sponge for a new one removes a working piece of biological filtration from a 4.3-gallon system, which is why the media page argues against replacing anything on a schedule. Replace it when it is falling apart, not when it looks brown.
What we cannot tell you
How much flow it costs you. No manufacturer in this category publishes a pore size or a flow-restriction figure, so there is no way to compute the effect on the turnover arithmetic the rest of this hub is built on. What we can say is the direction - it reduces flow - and that in a nano tank running above the 4x turnover this site targets, reducing it slightly is a benefit.
We also cannot tell you that these are better made than any other foam sleeve on the market. What we can tell you is that they are sold in four published intake diameters covering every filter we recommend, which in a category of indistinguishable unbranded foam is the whole basis for choosing one.
