Wet Thumb:  Marine Aquaria


"Ecology" is the science of the relationship of living organisms (animals, plants etc.) with their environment. It is important to the 'Native Marine Aquarist' because we do not have a shopkeeper to rely on to guide us on how to keep the various, fish, crabs, sea anemones and other life found at low tide around the British coast.


All good aquarists are responsible for the welfare of the creatures in their charge and they should strive to keep the fish and invertebrate animals in the best conditions as possible. He, or she, should endeavour to mimic the conditions that the fish would inhabit in the wild as far as it is practical.

Ecologists will then introduce other terms like the "niche" in which each species will occupy in the wild world. In its simplest terms "niche" can mean the physical space inhabited by a creature, but then other factors come into operation that restrict the distribution of species in the wild. These are of great interest to the aquarist when deciding the conditions in which to keep the fish or other creature. When the fish is kept in conditions just outside of the conditions in which it lives naturally, it is said to suffer "stress". This is rather a vague term and the symptoms shown by the fish include rapid breathing, gasping at the surface and other signs that resemble an infection.

Even in the sea, there are mass deaths when conditions become unsuitable for the survival of urchins and other sedentary creatures that are unable to move quickly enough to a more favourable area. I have also seen thousands fish of several different species spend over hour frantically jumping out of the sea when the water the water became unsuitable for their survival. Usually, they can detect minor changes in water quality and will simply swim elsewhere.


If a fish demonstrates a "stress" condition, the aquarist must act quickly. It is no good reaching for the medicine bottle and dosing the tank like a quack doctor from the nineteenth century. The best remedy is to remove the ailing fish to a favourable environment.

The following list is only a guide and is not meant to be a comprehensive troubleshooting procedure:


All life can only survive within a temperature range. Because water has a higher specific heat than air, it will heat up and cool down much more slowly. Consequently, aquatic creatures will inhabit a much narrower temperature range than terrestrial animals.

This is important to the aquarist who has to:
1. To keep the fish in the temperatures in which it is naturally found.
2. To avoid any sudden fluctuations of water temperature, or introducing any fish into temperatures of more than 2oC difference.

The aquarist keeping British marine creatures usually has the find some method of cooling the water during the summer1.


Marine creatures have to be very efficient utilisers of the very small proportions of the dissolved oxygen in the sea available. At the usual 100% saturation at 20oC, the amount in the open sea will be 7.4 mg/l (or parts per million). As the temperature rises, the amount of dissolved oxygen decreases.

The important factors to consider are:

1. Oxygen enters the aquarium water at the surface. Agitation of the water surface will increase the oxygen in the water. This is so important that this is the first thing to do if a marine fish begins to look ill.
2. Overstocking the aquaria is the commonest reason for an oxygen shortfall, as the inhabitants of the tank use up all the oxygen quicker that it can be renewed from the atmosphere and suffocate.
3. The biomass (all the living organisms consuming oxygen) in the aquaria increases with age. This includes the growth of the fish, but also the increase in bacteria and micro-organisms in the filter bed that consume the waste products and remains of the food that is introduced.


Although, feeding the sufficient amount of suitable food would seem to be obvious; failure to supply the correct foods can be responsible for the weakening of a fish over a period of time.

There is an interesting debate on this subject. One sensible suggestion is that it is a good idea to feed marine fish, freshwater foods and earthworms to avoid the possibility of introducing disease into the narrow confines of an aquarium. What could possibly be wrong with that?

Freshwater prawns and earthworms provide excellent food for most of the common British marine species, and may be the best and easiest way for the inland aquarist. However, this goes against the grain for me and there are several reasons:

1. I like to study my tank inmates in as a natural condition as possible, which means watching them search out and consume natural prey.
2. Disease is actually responsible for the deaths of less than 1% of marine fish, contrasted to malnutrition, temperature intolerance, oxygen shortfall and poor water chemistry.
3. If you live near the coast, the basic staples of mussels and prawns are plentiful and easy to obtain. Limpets make a poor choice and are only used as an emergency food when others are not available.
4. The fish simply may not recognise unfamiliar foods. Most rock pool fish eschew flake food. Seahorses and pipefishes will only take moving prey like mysids.
5. Some freshwater worms, brine shrimp and unnatural foods may not provide suitable nourishment. 


You will not become an expert aquarist just by reading this web page, as I have not included important factors like water chemistry, salinity, compatibility between species, lighting, culturing live foods for fry and specialist live feeders, and capturing and transporting live specimens.

The 'Manual of Fish Health' by Dr Chris Andrews, Adrian Exell and Dr. Neville Carrington (Salamander 1988) is recommended for its stated purpose of  'promoting a deeper understanding of fish, their environment and the diseases that affect them.'

For professional aquarists 'Dynamic Aquaria', by Walter H. Adey and Karen Loveland, is now in its second revised edition and published by Academic Press 1988. This book talks about 'Building Living Ecosystems that are sufficiently similar to their wild counterparts, so that the dynamic processes that occur in the wild can be studied.'


Ailments occur in wild fish from the shores around Britain like the Black-spot disease Cryptocotyle lingua, which does not spread from fish to fish. A large isopod (wood-louse type) crustacean called Anilocra is known to live on wrasse around the Channel Islands. Parasitic worms may inhabit most of the guts of some fish, but are rarely the sole cause of death. I never encountered the swim-bladder upsets that seem to be quite commonly found in captive-bred Goldfish.


1 Cooling the aquaria is usually achieved by using a 'beer cooler' fed by a powerhead, or by a special aquarium cooler.

Wet Thumb page

British Marine Life Study Society Home Page
News 2019
News 2018
Main Links
Membership Form
Top of the Page