How to modify a hang-on-back filter

The single most worthwhile modification. Most hang-on-back filters ship with a disposable cartridge (a thin sponge wrapped around carbon) that you are told to replace every month, which throws out your bacteria each time. Swapping it for reusable media filters better and protects your cycle.

How to do it

• Remove the plastic cartridge and its frame.
• Refill the chamber in the order the water flows through it: coarse sponge first, then finer filter floss (mechanical), then biological media such as ceramic rings or sintered glass like Seachem Matrix.
• Add chemical media last and only if you need it, such as a bag of Seachem Purigen or activated carbon.
• Use a media bag or a piece cut to fit so nothing tumbles into the impeller, and leave a clear path so water flows through the media rather than around it.

A sponge that slips over the intake tube. It stops shrimp, fry, and small fish being sucked in, keeps debris and plant bits out of the impeller, and adds biological surface area. It is close to essential for shrimp and breeding tanks.

How to do it

• Buy a pre-filter sponge sized to your intake, or cut a hole through a piece of coarse sponge, and push it onto the intake tube.
• Rinse it in old tank water during your weekly water change; it clogs faster than the main media, which is the point, as it protects everything downstream.

A pre-filter sponge is doing a small version of what a sponge filter does, which is why it adds biological capacity as well as protection.

A bonus: a spare pre-filter sponge running on your main filter quietly seeds itself with bacteria, ready to drop into a quarantine tank at a moment's notice.

Hang-on-back filters can push too much current for bettas, gouramis, and other slow, long-finned fish, and for shrimp and fry. A baffle spreads or slows the returning water.

How to do it

• The simplest baffle is a strip of sponge wedged where the water pours back in, which diffuses the flow.
• For more control, clip a cut-down food-grade container or bottle over the return lip to widen and soften the outflow, or fit a printed baffle.
• Aim the flow along the glass rather than straight down into the tank.

Do not block the outflow so completely that water backs up inside the filter and overflows; always leave a clear path for the water to return. If you suspect the filter is simply too powerful for the tank rather than badly aimed, the filter guide covers the turnover you actually need.

The trickling waterfall sound comes from the water level sitting too far below the return lip, so the water drops through air. It is an easy fix.

How to do it

• Raise the tank's water level so the return sits closer to the surface. This is the most effective fix, so keep an eye on it as evaporation lowers the level.
• Keep the filter box itself full and primed, so it is not gurgling air.
• A sponge baffle at the return also muffles the splash.

A sudden increase in noise is usually a clogged intake or a low water level, not a failing filter. Top up the water and clean the intake before assuming the worst.

A surface skimmer draws water from the very top of the tank, clearing the oily biofilm that builds up on a still surface and improving gas exchange. It is most useful in planted or low-flow tanks.

How to do it

• Fit a clip-on surface-skimmer intake if your filter offers one, or add a DIY skimmer box or an intake extension that pulls from the surface.
• Where you can, keep drawing from lower down as well, so you are not only skimming the top.

Keep the water level topped up: a skimmer stops working, and starts sucking air loudly, the moment the level drops below its mouth. Pair it with a pre-filter sponge if you keep fry, so they are not drawn in.

Most hang-on-back filters need water already sitting in the box to start. The impeller can push water along, but it cannot lift it up a dry intake, so a filter whose tank level dropped too far during a water change will hum without moving anything.

How to fix it

• Unplug the filter.
• Pour tank water into the filter box until it is full.
• Check that the intake tube, and any pre-filter sponge on it, is fully submerged.
• Plug it back in. It should catch within a few seconds.

Avoid it next time by keeping the water line above the intake while you work, or by switching the filter off before the level drops. The same restart applies after a blackout, which the power outage guide covers.

This is the most common side effect of the cartridge swap, and it is a plumbing problem rather than a pump problem. The pump delivers water at a fixed rate. If the media is packed too tightly, or has clogged since you packed it, the water cannot pass through quickly enough, so it backs up and takes the easiest route out instead.

How to fix it

• Take some media out. A chamber that is full but loose filters better than one that is crammed.
• Rinse the mechanical layer in old tank water. Fine floss clogs first and is usually the culprit.
• Repack so the water has a clear path through the media rather than around it, and leave a gap at the top of the chamber.

Bypass is the quieter version of the same fault. Instead of spilling over, the water slips through a gap between the media and the chamber wall, so the filter looks like it is running normally while much of the flow never touches the media. If the tank stays dirty after a media upgrade, check for that gap.

A rattle is almost always the impeller, the small magnet-driven rotor that moves the water. A lower buzz or vibration that you feel through the cabinet is usually the housing rather than the motor.

How to fix it

• Unplug the filter, then slide the impeller out of its well. On most models it pulls straight out.
• Rinse the impeller, its shaft, and the well it sits in. A grain of gravel, a snail shell, or a fragment of plant is the usual cause.
• Check that the shaft is straight and that the vanes and magnet are not chipped or cracked, then seat it fully before refitting.
• If it is a vibration rather than a rattle, make sure the unit sits flush and level on the rim and is not resting against the lid or a cable.

A worn impeller is a consumable, not a dead filter. It is also the reason the filter guide recommends buying a brand whose spares are stocked locally.

Work from the intake forwards, because the cause is nearly always upstream of the pump.

How to fix it

• The pre-filter sponge. It is meant to clog first, so rinse it first. Squeeze it out in old tank water.
• The intake tube. Look for a leaf, a snail, or a build-up of mulm across the slots.
• The media. Too much of it, packed too hard, is the usual fault after an upgrade. Fine floss sitting ahead of a coarse sponge clogs far faster than the other way round.
• The impeller. A film of mulm on the vanes cuts flow noticeably. Clean it as above.

Some loss of flow is normal. A mature filter runs a little slower than an empty one because the media is doing its job, so judge it against how the filter ran last week rather than against the figure on the box. The maintenance schedule sets out how often to clean what.