Hi we've just had to euthenaise (by putting into the freezer) our 7 year old fantail after he suddenly lay at the bottom of the tank on his side. At first it looked like he was sleeping but became obvious he was struggling to move so we decided the kindest thing to do was let him go peacefully. A month ago the one he had shared his tank with since we first got them when they were tiny also died- we weren't home when it happened but found him at the bottom of the tank on his side too. When that one died left it for a couple of weeks on case there was any disease, but the one we lost today seemed fine so we got him a companion, which is tiny, probably only a few weeks old. My little girl is really worried about the baby ("Noodle"!) in case he's going to die too..any advice? They've been in a bio orb but we took Noodle out earlier when it was obvious the other one was dying. He's currently in a very basic fish tank. There seems to be some negativity here about Boo Orbs, which I didn't know about before. Any advice about where we should house Noodle bow, what to look out for and when, if at all, to get him another friend would be much appreciated! I feel a bit daft saying all this about fish but they grew to be part of the family and my little girl will be really upset if the baby one dies too. Thanks, Sally

Euthanizing a fish by freezing is isn't really the most humane means of putting it out of its misery. While it does seem easy and apparently painless, it is actually quite the opposite where the latter is concerned. It's normally recommended to use a high concentration of Clove Oil in a plastic bag (or any other clean container) filled with aquarium water.

Seven years isn't bad for a goldfish, many don't make it past a few weeks. There's plenty of documented negativity towards Bi-Orbs (and similar 'overpriced fish bowls' from the manufacturer: Reef One) on the internet. This is mainly based on the mediocre and easily blocked gravel-based filtration system, their small size, the low surface area that bowl/cylindrical-shaped aquariums provide and also Reef One's marketing of its Bi-Orb/Bi-Ube 'tanks' for species of fish that are completely unsuitable for them.

For a single 'fancy' goldfish such as a fantail, I'd suggest a minimum of 45 litres per fish for the time being. This may seem excessive, however you have to take into account that the fish will continue to grow to an adult size of around 15cm/6"; and even then the fish will need a much larger tank. A 3ft tank should really be considered as a suitable minimum for two adult fantails - this will need to have adequate filtration to cope with the waste produced by these fish.

It would be wise to assess any potential causes for the deaths of the other two fish, to prevent a repeat from happening. If you access to a liquid test kit, I would recommend testing the aquarium water for pH, ammonia, nitrite and nitrate. Alternatively, take a sample of aquarium in a clean (and preferably sterile), food-grade container to an aquatic store for testing.

High levels of ammonia (produced by rotting waste and by the fish in metabolic processes; just like the ammonia found in human urine) could be present along with nitrite. Both substances are what can be described as toxic at concentrations above 0ppm (parts per million) or 0mg/l. (milligrams per litre) - the toxicity of ammonia is further increased as the pH and temperature of the aquarium water rises.

Other factors that need to be checked out include the use of chlorinated water in water changes. Chlorine, chloramine and heavy metals present in untreated tapwater are all lethal to fish.

Most fish die or become infected by an aquatic disease because of something wrong with the water quality or in the presence of a toxic substance that has somehow found its way into the aquarium. In cases of infection, poor water quality causes stress which in turn lowers the immune systems of any vulnerable fish.

The usual advice regarding tank sizes is: the bigger the better. Fishkeeping isn't as easy as it looks, and it's not exactly cheap. Do it right, and your fish could thrive to a ripe old age.