Saltwater fish stocking

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Great Lakes

Always do right, not popular...
May 11, 2002
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Southern Michigan
This is an attempt to make all newcomers to the marine world understand why marines stocking levels are significantly smaller than tropicals.

I am not a marine biologist, so I will start this out with what I am able to. When I get some additional imput, I will turn it into an article giving all contributors credit.

Freshwater and marine fish differ greatly in the way they utilize their environment,(water).

A freswater fish will ingest small amounts of water with its food and various activities. Its body will also absorb water to aid in its hydration. Excess water is then expelled with the urine. The impact this physiological behavior has on stocking levels is twofold. The minimal ingestion and absorbsion of water limits the health hazards of toxins in the water, and the water in the urine helps dilute it making a minimal impact on its surroundings.

A marine fish, for the most part, is the direct opposite. It ingests large amounts of water continuously. And instead of absorbing water to aid in hydration, the marine fish expells water from the inside out. And since it flushes its cells with water, very little of it is used in the digestive tract, making a marine fishs urine very concentrated. The twofold impact here? The marine ingests so much water during its day that the risk of toxins getting into its system are far greater, and their urine can have a greater effect on its immediate surroundings and the overall parameters in the tank.

So in summary, you can see how the Marine fish is practically a filter regarding the way they ingest large amounts of water and flush it through all their cells. This is why water quality is so very important in the marine environment.

I hope this helps many of you. Knowing or being told what appropriate stocking levels are is one thing. Once I understood why, I have a lot more respect for the health of my fish and would not dream of having a crowded tank.

Cheers, GL
In addition, the territorial needs and behaviors of most marine fish are vastly different than that of FW fish. The biological aspect can be compensated for (increasing filtration, increasing water change frequency, chemical filtration, skimming, etc), but the physical space needed by the fish to feel safe is limited by the size of the tank. Obviously, in the ocean, there are multiples of all the fish, and while there is some fighting and chasing, it seldom results in deaths. This is because the fish being chased has a number of escapes--hiding in rocks, leaving the other fishes territory, etc. In an aquarium, escape options are limited, and the chased fish often can not escape, so is harrased to death. For stocking, this means understanding the physical territory needs of the fish and addressing that by having the appropriate number of fish per tank--even if that means having just one.

Further, many freshwater fish occupy waters that fluctuate in size. During drought seasons, some water can be reduced to half or less their maximum volume. This results in several changes to the water--chemistry can change, temperature can rise, and many pollutants become more concentrated. The limited space available also impacts FW species. As a result, they are well adapted to handling brief (in terms of the fishes lifespan) periods of crowding and inadequate water conditions. They can tough it out, and deal with the changes. Some can tolerate inadequate space for seasons without being physically impaired, or stunted. For marine animals, these fluctuating conditions are seldom, if ever encountered. The ocean is huge, and very, very stable. Salinity fluctuates around some areas (FW rivers coming in, FW springs, rain, etc), but the overall impact of these events is localized and quickly diluted, never resulting in serious changes to the watr chemistry. The same applies for most fish/coral wastes--they are very minor in comparison with the overall volume of the area. Since the ocean never diminishes seriously in size, the ability to stop growing when the physical space becomes limited simply does not exist in marine animals, as it does in many freshwater species.

While 'rules of thumb' are desirable, they really are meaningless when dealing with marine animals. Even the much quoted 'rule' for FW fish applies only to small, slim bodied fish such as tetras--and with the huge diversity of shape and size in marine fish, it becomes even less meaningful. So, when selecting stock, here are the guidelines I use:

1. Adult size. The adult size of the fish should be no more than 3/4 the open water in your tank. This means if you have a 55, that is 12 inches front to back and 4 inches of that is taken up by rockwork, you can have a fish that gets up to 6 inches easily. There are obvious exceptions--eels will exceed the front to back dpeth of most tanks, but are very flexible and like tight spaces, so aren't limited in this way.

2. Body shape. If a fish is boxy, like a puffer, it will produce more waste than a chalk basslet. The more mass the body has, the more tank volume you will need to support it. Always use the adult size for making this decision, and you will be better off giving it more space than it needs than too little.

3. Temperment. Some fish will not tolerate tankmates for long. Some will put up with them as juveniles, then change overnight into viscious killers. Never assume the fish you get will be the exception to the rule. Clown triggers are known for this--if you get one, plan on it happening eventually, and stock accordingly.

4. Predation. Predators will eat prey animals. They may co-exist for a short time, but do not rely on this being a long term situation, and avoid the odds of having your fish eaten.
Excellent points Puck.

I was so focussed on the physiological reasons, I neglected the habitat angle.

There is another way to determine how many fish go into your tank. This is a rule that most reefers use. Open your wallet while looking at all the fishes, and say "awe shucks". :lol:

lol. I have hasd my tank for a week.

It contains 20 kg of live rock and a toadstoll coral. It ia 190 litres. What fish would I be able to add tomoroow
What Puck said about Stability in the ocean and polution dilution is totally correct and very important. Now I'm going to go over what GL said in more detail, Just because I apperently have more free time than he does, and less of a life. useing alot of keywords so you can google for more information if you so wish.

When water is contained in a semi permiable membrane like the fluid mosaic lipid bilayer that covers cells diffusion and molecular speedcombined with probability move what can be movedaccross the membrane down the concentration gradient meaning it goes from where there is a high concentration to a low concentration. In the case of water this process is called ossmosis and the concentration of water in fresh water is higher than that in salt water.

In fresh water the content of desolved compounds in side of the cells of the animal is far greater than that outside in the environment, and the blood is approximately the same (but not quite) as the cells and interstitial fluid (the fluid inbetween cells) are now this causes water to diffuse into the fish. If this were allowed to run rampant the fish would swell up to an enourmus size and this does happen in the case of dropsy. Withing the kidneys the blood is pushed throught something called a glomuerulis, it is esentially a filter which keeps in the proteins and red blood corpuscles and forces out glucose, salt and waste products as well as toxic substances that may have been consumed during eating or been absorbed during the respiration processes, The blood plasma is then passed throught a tube (the tube of henly) designed to grab what the fish wants and lose what it doesnt, in fresh water it saves salts and glucose and tries not to kjeep wastes toxins and water Now this is a fairly easy task for a fresh water fish so the tube isnt to long.

Now in salt water fish osmossis takes water out of the gills and when left unchecked the fish would litterally die of dehydration just like we would if we drank sea water. to counteract this the salt water fish is constantly drinking as water is lost throught the gills an equivalent amount of salt must be lost so saltwater fish use there considerably longer loops of henley to squeez out a very strong urin in skanty amounts, this taxes salt water fishes kidneys a great deal and salt water fish have more powerful kidneys to deal with the problem but they are more dependant on them and toxins that effect the kidneys have a more harmful effect on salties than fresh water fish. OF course both types of fish take in large volumes of water but the fresh water fish is haveing its kidneys flushed by water constantly where as the gills of a saltwater fish are being flushed constantly, and the gills don't really need it like the kidneys do.

Also the urinary system isnt part of the GI tract, it actually develops from a different part of the system (mesolemma rather than endolemma I believe) and the GI tract carries out undigested food where as the urinary tract carries out the waste products of cellular metabolism and any toxins that may have leaked in, Common misconception; Oh and fun fact the reproductive orgains acctually come off the GI tract in one out of every 100 million live births in human males the testes are acctually not partitioned from the lower intestines :( , thank god thats not common.

Bottom line fewer fish => better coloration and longer lives

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