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Difficulty is a reference to how easy or difficult a fish is to keep.
These are fish that are effortless to look after and a perfect choice for beginners.
These are aquatic animals that are easy to keep in an cycled aquarium tank. The animals listed here are well able to tolerate some of the basic mistakes of a beginners tank.
They are hardy animals that can withstand a wide range of pH or a small amount of ammonia or nitrite and can adapt to a wide range of GH and KH.
They are undemanding in their food requirements and are therefore cheap and easy to feed.
These are aquatic animals that are moderate to keep in an cycled aquarium tank. The animals listed here are relatively hardy, but may have some requirements that means they either will not tolerate flucuations in their tank environment, need careful consideration with tankmates or need specific foods.
These would be best kept by those with some experience in fishkeeping, or by beginners after extensive research.
These are aquatic animals that are challenging to keep in an cycled aquarium tank. These fish will probably be hard to find and expensive to buy. They may grow large or be rare in the aquarium trade. They may also be fussy with water conditions and food.
Creatures in this category should only be attempted by experienced fishkeepers. This may be because the creatures are high maintenance and need immaculate water conditions, grow large, or are not sociable with other fish or invertebrates.
These creatures are classed as "impossible" as they may grow to an extraordinary size, need exceptional water conditions, or simply not adapt to life in captivity at all.