Rock pool fish in an aquarium
Rock pools are very exposed to the elements, most are quite small and subject to huge changes through the day when the tide is out. The sea on the other hand is remarkably stable. There are all kinds of factors which can quickly change the environment in a rock pool, a sudden downpour, a bright sunny day, snow and wind.
The fish which inhabit this really quite hostile niche have adapted to do so or quite simply they wouldn't be there.
There are two main groups of animal which are likely to be found in rock pools, the ones which really live there on purpose and the accidental stranded there. It is important to know the difference between the two because they require different care.
The animal life which has made rock pools their home on purpose have to be able to cope with change, sudden change. On a hot summers day a shallow rock pool will heat up quite quickly, water will evaporate making the pool saltier and then when the tide returns the pool will instantly cool by a substantial margin of 10°C or more. Rain will dilute the seawater and again there will be a sudden change back when the tide returns.
True rock pool animals can cope with these changes quite easily.
Animals of the second group which occasionally find themselves stranded in rock pools normally live in the shallow sea where the environment is a lot more stable, as a result they are far less adaptable and can't cope with the same extremes.
Resident intertidal species are well adapted to living in the harsh environment between the tides. Salinity, temperature, pH and oxygen levels are likely to swing quite dramatically and suddenly on an almost daily basis. Most intertidal resident species found around the UK will live quite comfortably between the temperatures of 9 - 23°C in Summer the temperature often rises above the higher of the two temperatures for the time that the tide is out, at the higher part of the intertidal zone this could be last up to 8 hours but the fish which live at the higher part of the intertidal zone tend to be the most tolerant and they can tolerate temperatures in to the low thirties centigrade. If the temperature rises to 35°C then even the most tolerant species will be at its limit and less tolerant species will almost certainly die.
Anything found stranded at the very low tide mark may be a shallow sea inhabitant and won't be able to cope with conditions which vary to widely from its normally quite stable environment. If you keep anything from this area you will almost certainly need to keep conditions in an aquarium the same as the shallow sea. The ideal upper limit for most species from the shallows should be in the region of 15 - 18°C . This means an aquarium cooler is almost advised if the creatures in your care are to thrive.
The cheapest method is to use fans specially designed to fit in the hood of the aquarium. These can reduce the temperature by as much as 7 or 8°C when used correctly and cost in the region of £20.00. There are dedicated aquarium chillers available which work in a similar way to a fridge but where the temperature can be set to a specified level, but these systems are very expensive and cost several hundred pounds.