Marina I25 Internal Filter

Filtration in a small tank matters more than in a big one, not less. There is less water to dilute anything that goes wrong, so a 25 litre tank swings faster and further than a big one — which is exactly why the small tanks sold as beginner set-ups are the hardest to keep stable.

The Marina i25 is an internal filter for goldfish or tropical tanks up to 25 litres. It runs three stages of filtration through a single Power Cartridge, which is included: water enters the centre of the cartridge for mechanical, chemical and biological filtration and exits through a polyester foam layer, leaving the trapped debris inside. The motor sits at the bottom for quiet running, and there is a waterfall spout that oxygenates the water continuously as it returns.

• For goldfish or tropical tanks up to 25 litres
• Three-stage filtration: mechanical, chemical and biological
• Power Cartridge included
• Bottom-positioned motor for quiet operation
• Waterfall spout for continuous oxygenation

Never rinse a filter cartridge under the tap. The biological stage is a colony of bacteria living in the media, and chlorinated tap water kills it — which turns a working filter into a sponge overnight and is the single commonest reason a tank crashes after a clean. Rinse media in water taken out of the tank, and if you are replacing a cartridge, run the old one alongside the new one for a couple of weeks so the bacteria transfer. A goldfish also outgrows 25 litres, so treat this as a filter for a small tropical tank or a temporary one rather than a permanent goldfish home.

Who it's for: small tropical tanks, quarantine and hospital tanks, and anyone replacing a noisy or failing internal filter.