They decided many years ago that four inches by four inches of surface area per inch of fish was the correct formula in a tank without aeration, and this still stands today, even with all the technology that is available.
A four inch by four inch square of surface area per inch of fish is the minimum requirement, so if your fish is say four inches long, it would require a surface area of 16 square inches multiplied by four giving you 64 square inches of surface area to be on the safe side. So by the above guideline you can see that you would only be able to place two fish of four inches long in an 18x10x10 tank, as a third fish of this size would be just pushing the limit a bit.
If you estimate the length of your fish, not counting the tail, and use the conversion above, then you will not be far wrong, however, you need to consider several things when doing this. The main thing to consider is of course that the fish may not always be the size they were when you first bought them, and may soon outgrow the area you have allocated for them. Placing ten or fifteen Neons in say a two foot tank would possibly be acceptable, and the same would apply to fifteen baby Swordtails, but obviously, the Neons will reach just over an inch when full grown, whereas the Swordtails can in some cases get to four inches an more, so this should be considered first.
The stocking levels of a tank can vary also in the example above, as a 24x12x12 wide tank will hold less fish than a 24x12x15wide tank. This is because the 24×12 wide will only give you 288 square inches of surface, whereas a 24x15wide will give you 360 square inches of surface area to play with, and therefore you can add a few more fish. The depth of a tank has little effect on the amount of fish you can keep, within reason of course, but the surface area is the thing that matters mostly. With the addition of aeration and filtration your fish levels may be increased, but still we will reach a level where the waste products produced by the fish will outrun the capabilities of the filter. When this happens we have a buildup of ammonia, plus the nitrites and nitrates will climb, which will all lead to problems. Running a highly stocked tank is a gamble at the best of times, even with the best of filtration, and the water conditions will need to be at their optimum at all times, which may involve partial water changes several times a week, and regular checks for ammonia buildup.
Some early signs of an overstocked tank are fish getting “left behind”. By this you may see runty unwell fish, or even emancipated fish that rarely get a chance to feed in peace, so they are pushed to one side by the others, and soon become unwell, and possibly die. Fish lurking in corners with clamped fins are usually early signs of a problem. Other signs are gasping at the surface, or congregating on the bottom. The former is lack of dissolved oxygen, and the latter could be due to the metabolism of the fish being so low that it hasn’t the strength to act in the normal manner. Red or inflamed gills or body parts are another sign of ammonia buildup, as the ammonia literally burns the mucus from the body of the fish leaving it open to infection. Once you fish become rundown, then problems can spread rapidly though the tank causing sudden deaths that appear to happen for no reason, but there is “always” a reason for a fish dying, and some of the reasons are above.
We have situations where we followed the basics and set our tank up, then after a given period we began to add fish and plants, and in the following weeks everything is looking good and healthy, but then the day arrives that you decide to add more fish to your stable setup. A quick trip to the LFS (Local Tropical Fish Shop) and you are faced with an abundance of fish that you would “just love to see” swimming in your tank, and in many cases you may consider the types you already have, along with the water conditions and space that they live in, or if they are compatible with your other fish… and also that you pick only the healthiest looking specimens you can see in the shop tanks, but invariably you don’t, and in many cases buy more fish (or too many extra ones) on impulse. The choices you make at this point are the ones that will either “make” or “break” your established setup. So you arrive home, full of excitement, and longing to see the new arrivals swimming free. We have all reached this point at one time or another, and thought little about our actions at this point, as we were deterred by the pleasure of the moment. At this stage you only have the single tank, so quarantine is out of the question for the new fish, so you float the bag for a while, then gingerly empty the fish into your precious established water. The roulette wheel has just been spun, and you have just placed your bet, and the gamble has begun. The “gamble” is unavoidable, if you want to increase your fish stocks, and your “established” tank has now become your “quarantine” tank, as you have now added an unknown element into the environment, both in the fish you added, and the water they were contained in. The fish at this point may seem a little stunned, but soon start swimming around and mixing with the other occupants, so you sit back and watch your new arrivals. Presuming you took into consideration about the amount of extra fish you can add, and the types, plus the water conditions, then at this stage you perhaps feel pretty confident that you have done the right thing… and you may have,… but over the next ten days or so you “could” be in for a few moment of anxiety as the fish begin to show various signs of illness, and some may even die for some unknown reason.
If this happens, then several things may be the cause, along with all those mentioned above.
Fish… much like us,.. have the ability to carry a disease but not show any symptoms, and a seemingly healthy fish could be harbouring a parasite that you can’t see, as it might be internal, or in its early stages of development. The water you added from the fish you bought could have been full of microscopic nasties that took several days to manifest themselves, or a fish that you bought might just have rejected the new conditions it was placed in, so suddenly became ill and infected the others. There could also be a “dominant” fish in the ones you bought, which, unbeknown to you, has been hassling the other fish, or preventing them for feeding freely, which again has led to stress of one or more fish, which has brought on the sudden outbreak of illness. There are so many factors here that to isolate them all would take many pages like this one, and even if you follow the strict quarantine rules, you could “still” get some of the problems mentioned above. Usually the way out here is to “blame the LFS”… but invariably they are “not” to blame, and provided the fish you bought were healthy, and the tank you bought them from was looking healthy (and possibly still is), then the fault may be your own.
So Do We Have An Answer..?
You can only fit so many fish into a given space, and “any” increase in fish stocks should be done in small stages, and with great care. Just as with our roulette wheel… a small bet means a small loss, and a large bet can leave you broke. In a way it is the same with your fish, as adding a couple of fish per week can give you more control over what is happening, rather than dumping six or more newcomers into an established tank. As above… the newcomers may show no signs of problems, but they “could” infect your existing fish and leave you with a tank full of trouble. These newcomers need watching “very” carefully for “any” signs of illness or incompatibility with the others, and at the first signs of problems they should be removed and isolated from the rest of the tank inhabitants. Adding just a couple of fish each week will “still” change your tank conditions, but it will do this “slowly”… whereas placing a larger number of new fish in your tank all at once will change the conditions “rapidly” and cause possible stress to the fish through extra ammonia levels. This ammonia addition may seem insignificant at first, but as the days pass, these levels will grow, as will the problem.
So many fishkeepers jump into this trap and overstock their tank in the early stages, and by the time they decide to buy another tank to cope with the overload, it is usually too late and they have to start all over again. Your over enthusiasm to “get things going” will come to an abrupt halt if you don’t have the patience to build things up in stages over a period of several months, rather than a period of several days. Do it right… get it right, and take your time, and your patience will be rewarded.