FAQ: Filters for fish

Bettas have long fins and live in still water in the wild, so they need a gentle flow. A sponge filter or a small filter with adjustable flow suits them, with the outlet aimed so the surface is not churned.

In a shrimp tank the key is that the filter must not suck in the babies, so a sponge filter is most common, or a fine sponge is fitted over the intake tube. Shrimp also graze on the sponge surface, feeding on the microbial film that builds up there.

Fit a fine sponge or a special mesh guard over the filter intake, reduce the flow, or raise the fry in a separate tank with a sponge filter. The sponge on the intake needs rinsing more often, as it clogs quickly.

Cichlids are often large, eat a lot and dig in the substrate, so they need a powerful filter with plenty of mechanical and biological capacity. External filters or sumps are most common, as cichlids can dislodge or damage internal ones.

Aquatic turtles foul the water much more than fish of the same size, so a filter rated for two or three times the tank volume is usually chosen. Because the water level is often low, an external filter or an internal one suited to shallow water is convenient.

An internal filter is fixed to the glass with suction cups so that it is fully submerged and the outlet sits just below the water surface. A corner or a spot behind decor is best, where the flow can spread across the whole tank.

Hardened or dirty suction cups are the usual cause. Clean them and the glass, soak the cups in warm water to soften them, and replace worn ones with new ones, which are sold as spare parts.

Most internal filters must be fully submerged, otherwise the pump draws air, gets noisy and can overheat. Special filters that work at a low water level are made for shallow tanks, such as turtle setups, and this is stated on the box.

Many internal filters have a tube that draws air into the outlet stream, adding oxygen to the water. A small valve sets the amount: if the bubbles are noisy or a planted tank loses too much CO2, you can reduce or close the aeration.

Place the filter under the tank, below the water line, connect the intake and outlet hoses, put the intake near the bottom and the outlet at the opposite end. Before switching it on, fill it with water as the manual describes and check that the connections are tight.

Many external filters have a manual priming button that pulls water into the hoses and canister. If yours does not, start a siphon on the intake hose or fill the canister by hand before connecting it, and only then switch it on.

External filters are designed to have water run into them naturally, as a siphon. If the filter sits higher than the tank, the pump struggles to draw water in, air collects, the filter gets noisy and may fail.

The usual causes are a badly seated or dried-out lid seal, loose hose connections or a cracked hose. Check the O-ring, grease it with the lubricant the manufacturer recommends or plain petroleum jelly, tighten the fittings and watch for drips for a while after each cleaning.

The taps shut off the water so you can disconnect the filter without draining the hoses. This makes cleaning much easier: water does not spill on the floor and the filter restarts more easily afterwards. Some people also use them to adjust flow, but closing them hard can make the pump overheat.

It is a tube with a row of holes that connects to the filter outlet and spreads the flow more widely and gently. It is useful in long tanks and where calm-water fish live, and aimed at the surface it also adds surface movement.

It is a mesh or sponge over the intake tube that stops small fish, shrimp, snails and larger debris being sucked in. A sponge also catches dirt before it reaches the filter, so the filter itself clogs more slowly.

A surface skimmer draws water from the very top and removes the oily film, dust and protein layer that collect there. It comes as a separate device or as an attachment on the filter intake, and a clean surface improves gas exchange.

Slime and algae build up inside the hoses over time and reduce the flow. Every few months, when cleaning the filter, run a long hose brush through them, and a sponge on the intake helps keep less dirt from getting in.

The hose diameter must match the filter's fittings listed in the manual, since a narrower hose cuts the flow and a wider one fits poorly and leaks. When replacing hoses, choose flexible, kink-resistant ones made for aquariums.

The cabinet needs room not only for the filter but for hoses without sharp bends, the taps and space to lift the lid when cleaning. Before buying, measure the filter's height with the lid open and leave room for your hands.