It all depends on how you define "hardy".
Do you mean adaptable, or able to tolerate water quality problems, or forgiving of what we might think of as beginner's mistakes? There are plenty of brackish water fish that are supremely adaptable. To take one example, the desert goby,
Chlamydogobius eremius. It will live in anything from neutral freshwater through to water twice as salty than the sea; it is able to survive very low oxygen concentrations; and it can tolerate temperatures from just above freezing through to well over 35 degrees C. Another example from the marine side of the hobby is the puffer
Arothron hispidus, which though normally sold as a marine fish, will tolerate freshwater conditions for months without harm, especially when young.
In terms of being able to tolerate poor water quality, there are few fish that actually thrive in such conditions, but things like mudskippers and synbranchid "tulip eels" are notable for their tolerance of ammonia as well as being able to breathe air. Such fish would, by any standards, be considered very tough. Other fish with this sort of tolerance for sub-optimal conditions would include tench, some of the snakeheads, and certain catfish, notably
Hoplosternum.
If you want fish that will sail through the cycling process, and put up with slight errors in terms of pH, nitrite, and so on, good beginners fish would including things like zebra danios and bronze
Corydoras, both of which are rightly popular among aquarists. Even though most people keep them warmer than they'd like, these fish still manage to thrive. They are also easy to feed, easy to breed, and largely disease-free.
If you look at
this month's TFH, I have an article about "alternatives" to popular -- but difficult to keep -- community fish. Neons for example can be quite long-lived for their size, assuming reasonably soft water, and more importantly, that you buy from a batch that doesn't have Neon Tetra Disease. Other fish I'd tend to recommend against include ram cichlids and dwarf gouramis, both of which are particularly disease prone nowadays, even if once upon a time they were quite sturdy little fish. To be fair on the ram cichlid, it's actually quite robust if kept in very warm, very soft water -- it's just that most people don't keep them that way, hence their diabolical track record.
Cheers, Neale
