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by Andy Horton

Applied knowledge of the WATER CONDITIONS in which British marine fish and invertebrates live in order to provide a satisfactory home in aquaria.

Whatever branch of marine aquariology that you specialise in, it is essential to provide the water and other conditions that are as near as possible to those found in the wild.

 Fish and invertebrates found in British seas often can tolerate only a narrow range of conditions. The variable factors that are likely to be of most concern to aquarists are:

1)  Temperature.
2)  Ammonia content
3)  Salinity
4)  Oxygen (the amount of dissolved oxygen in the water)
5)  Other factors: nitrite, nitrate, pH., dissolved organic carbon (DOC) etc.


 50% of the fish and invertebrates commonly collected from British shores are unable to tolerate the sustained high aquarium temperatures of the Summer of 1989.

 In order to guide the aquarist to which species can be kept successfully, it will be necessary to research into the distribution range of each specie. It may help to divide the species into 3 groups based on sea temperatures:


This fauna includes fish and invertebrates found in the Arctic and North Sea. Typical species of interest to the aquarist include the Viviparous Blenny (=Eelpout), Zoarces viviparus, the Scottish Bullhead (=Bull Rout), Myoxocephalus scorpius, and one of the Spider Crabs, Hyas araneus.

Purple Sunstar      The Purple Sunstar, Solaster endeca, is an Arctic species

 Also included in this fauna group are creatures that reach their most southerly limit of distribution in the English Channel, where the maximum surface sea temperature may reach 17ºC, or 19ºC in inshore waters. They  include the popular Dahlia Anemone, Urticina felina (=Tealia felina), the abundant Butterfish, Pholis gunnellus, the Lumpsucker fish, Cyclopterus lumpus, the attractive Sunstar, Crossaster papposus, and the orange fish with the common name of Montagu's Sea Snail, Liparis montagui. The Hooknose, or Pogge, Agonus cataphractus, from sandy shores is also an Arctic fish.

 If these species are to be kept successfully, some form of cooling the water is absolutely essential.


This fauna includes fish and invertebrates found around the British Isles with their most southerly biogeographical range in the western Atlantic; but absent from the Mediterranean sea. Species found on the shore throughout the Summer like the Blenny (=Shanny), Lipophrys pholis, can survive and breed in aquaria.

Common Hermit Crab

 However, many of these species although found on the shore and in pools, like the 5-Bearded Rockling, Ciliata mustela, the Bullhead (=Sea Scorpion), Taurulus bubalis, the Common Starfish, Asterias rubens, the Common Hermit Crab, Pagurus bernhardus, the Plumose Anemone, Metridium senile, the Shore Urchin, Psammechinus miliaris, and many others, are more common offshore during the summer. They can be kept in the larger aquaria (over 100 gallons), and many of them are kept successfully in Public Aquaria, and in cellars and insulated garages where the temperature in the large volumes of water does not exceed 19ºC. Even in an unheated room, temperatures in the smaller tanks may reach 25ºC or more; and to ensure success, an efficient method of cooling the water is necessary in the very hottest weather that occurs every year.


The fauna of the Mediterranean, reaching the most northerly point of distribution in the English Channel and the western coasts of Britain and Ireland, is more varied in the number of species, and prove to the ideal fish and invertebrates for the home aquarist. In most years, the fish and invertebrates will be absent from the North Sea.


A small Ballan Wrasse, Labrus bergylta, is a southern species. It will grow too large for a 50 gallon aquarium in 3 years.

 The most popular species kept in aquaria include the Rock Goby, Gobius paganellus, the charming but aggressive Tompot Blenny, Parablennius gattorugine, the Snakelocks Anemone, Anemonia viridis (=A. sulcata), the Cushion Star, Asterina gibbosa, the Cornish Sucker (Clingfish), Lepadogaster lepadogaster, the Corkwing Wrasse, Crenilabrus melops, the Hairy Crab, Pilumnus hirtellus, and many others.

 Cooling the water is not necessary, and some of the creatures may actually die if the water gets too cold. i.e. below 9ºC.
 An experienced naturalist will not be surprised if some of these creatures do not fall neatly into these categories. The Black Goby, Gobius niger, other fishes of the same family, and the large Edible Crab, Cancer pagurus, could be described as Lusitanian-Temperate. Red Beadlet Anemones can be found in both Arctic and Tropical waters.

 Marine biological treatise and text books of the seashore attach great significance to the ecology of the pools themselves. Twice daily, inhabited pools will be renewed with fresh seawater by the incoming tide. The pools the furthest of the shore exposed to environmental factors leading to a marked deterioration of quality. These can include:

1) Rapid increases of temperature in pools up to 24ºC in Summer.
2) Evaporation of the water increasing the salinity.
3) Dilution of the water by rainfall.
4) High levels of toxic ammonia from decaying organisms.
5) Falls in levels of dissolved oxygen for several reasons.

 This has led aquarists to wrongly conclude that because rockpool fish and invertebrates endure harsh natural conditions, they can suffer similar conditions in aquaria. The major error occurs because of the description of what actually happens is inaccurate. Extensive observations on the shore indicate that all of the mobile animals of most interest to the aquarist will swim, crawl or float (sea anemones) to the more favourable offshore waters, or to the pools near the low water mark that provide a more stable living environment. During 'neap' tides in  the middle of Summer, many of the mid-tidal pools will contain relatively few examples of life. The same pools will be teeming with prawns and small fish around the time of the autumnal equinox.

 British sea life aquarists can observe the increased metabolic and breathing rate of the fishes as the temperature increases. For a few hours, this is not harmful as these conditions are likely to occur in pools until the arrival of the incoming tide. However, when a fish experiences sudden rises, or temperatures exceeding those to which it is able to physiologically adapt, it demonstrates symptoms that indicate that it is not receiving sufficient oxygen. This is known as the 'escape response'. It may gulp at the water surface, or even jump out of the tank. Identical behaviour is often demonstrated when levels of ammonia reach toxic proportions. In the latter case, the breathing tends to be more irregular. If a fish is introduced into water that is too warm, it is likely to die in a few (1 to 6) hours.

     Fish found in rock pools are often those which have a large distribution range. It is easy to understand that a fish like the Rock Goby, Gobius paganellus, which can be found in the Mediterranean, is happy to live in British coastal pools during the Summer. In an earlier article I explained about the tendency of the Common Blenny, Lipophrys pholis, to leave the water during hot spells. This qualifies it as the only European fish that has ethologically adapted to the seashore and rockpool habitat. All other fish swim into deeper waters offshore if the temperature or other conditions become unfavourable.

     It is interesting to note that feeding rarely occurs in the still waters of the pools, and that one of the first indications that a fish is suffering from heat stress, is its refusal to feed. This is most clearly seen in the Bullhead or Sea Scorpion, Taurulus bubalis. Unless the temperature can be reduced in the next 12 hours, or less, the fish should be returned to the sea. Rockpool fish can understandably tolerate rapid falls in temperature from 24ºC to 17ºC. It is not recommended in aquarium conditions.

     With the possible exception of the Blenny, none of the British fish seem to be able to permanently acclimatise to temperatures more than 2ºC above the maximum temperature of the seas in which they are found. (This may be as high as 26ºC for the Corkwing Wrasse, Crenilabrus melops, or as low as 17ºC for the Butterfish, Pholis gunnellus). Symptoms of intolerance to high temperatures by invertebrates is much harder to detect. Refusal to feed is often the only sign. Dahlia Anemones, Urticina felina (=Tealia felina), and Plumose Anemones, Metridium senile, are two vulnerable species.


The Sole, Solea solea,  is a warm water fish that grows far to rapidly for more than a temporary stay in a home aquarium

     Fish experience stress when the water becomes too cold. Black Bream, Spondyliosoma cantharus, demonstrate the opposite metabolic affects, slowing down their breathing and lapsing into a coma below 10ºC. (Observation in one specimen only.) A fall in temperature of 6ºC overnight resulted in visible signs of stress in the Grey Mullet, Chelon labrosus, including reddening underneath the fins.

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