The First Aid Kit treats a range of common fish diseases so is useful if you are not sure what your fish is suffering from.
To treat fish for illness usually involves working out dilution rates, carefully measuring out liquids and removing any chemical parts your filtration system. With this Kit you can simply replace the existing filter cartridge for the First Aid Cartridge and add the treatment sachets on the days instructed.
Poorly fish often means that the water is unhealthy, this is almost always caused by overloading the filter - feeding the fish too much or introducing fish too quickly are the most common causes. The best way to keep the water and your fish healthy is to minimise feeding and carry out regular maintenance.
Fish almost always eat far more than they need so it is very easy to overfeed them.
This is a treatment cartridge, it is not meant to be used long term, it will not work as a filter and will not prevent your fish from becoming ill. Only ever use treatments if your fish are ill.