Some other coldwater freshwater fishes appear irregularly in the pond and aquarium trade, beyond domesticated goldfish and koi varieties. Whilst many of these lack color, they may have bags of character, and interesting behaviors. One such fish is the miller's thumb or European sculpin, Cottus gobio. This fish normally grows only to 10 centimeters or 4 inches, and sometimes quite larger, to 18 centimeters,or a little over 7 inches. Even this is not huge, so they do not require raised indoor ponds or other massive aquariums. From the perspective of European aquarists, this is a true native fish, having a natural distribution over a swathe of Europe, including its presence on Great Britain.
Their habitat preference is for well oxygenated streams and rivers, with coarse beds featuring large stones. Although these sculpins are found in rapids, they have a preference for moderate flow. It is their behavioral selection for useful slack water refuges, and obstacles to uncomfortably high flow velocity, that enables their presence in faster flowing, upland headwaters. Younger fish prefer shallow, stony riffles, whereas the adults have broader habitat preferences, and can be found present in the wild around submerged wood and vegetation, in lakes and in slower flowing waters with softer bottoms.
Sculpins are a part of an important radiation of fishes known as cottoids, that also includes the lumpsuckers and greenlings, among some others. Traditionally the cottoids were regarded as a subclade of the scorpaeniform fishes, but the characteristics supporting that 'order' or 'suborder', were few and problematic, for example, their spiny cheeks. Cottoid fishes have turned out to be more closely related to the sticklebacks and eelpouts, than they are to the core scorpaeniforms, or the true scorpionfishes and their allies, which sculpins can certainly resemble.
Cottoids or sculpins are benthic fishes of marine to freshwaters, that lack a swim bladder, which is not unusual for fishes that live on the bottom, because it is a floatation device. C. gobio tolerates broader water requirements, than is often said to be the case. The pH of the waters they inhabit in the uplands is around neutral, but they can migrate down to lowland chalk streams, where the pH is 9, so these are fish of hard and base freshwaters. Although this species can be found in the low salinities of the eastern Baltic Sea, no elevation of specific gravity is necessary in the aquarium, and they are basically freshwater sculpins with a limited tolerance for low end brackish conditions.
Much is said online about the supposed demands of these sculpins for very cool water, and I have read the curious assertion that thejuveniles are more heat tolerant than the adults. Contrarywise, juvenile C. gobio are in fact less tolerant of higher temperatures, than are the adults of their species that have migrated away from riffles, and they become thermally stressed even below 20 degrees. The range of water temperatures at which mature C. gobio feed is as low as 5 degrees and also as high as 26 degrees centigrade.
C. gobio basically have a staple diet of arthropods. Older sources overstate the extent of their predation on fish eggs and fry, though with that said, they actually do consume such items opportunistically. Sculpins such as C. gobio are able to ingest large food items, and smaller ornamental fishes may end up as prey. C. gobio are territorial and aggressive among themselves, and prone to conflict with other bottom living fishes. Conflicts can be minimized by providing these fishes with a complex aquascape, in which lines of sight are easily broken. Stones, gravel, wood, and live water plants, are all appropriate for this species Sculpins are normally ignorant of dissimilar fishes with different patterns of spatial use.