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There seems to be a lot of bad information and bad ways of thinking about stocking an aquarium. The most common mistake and way of thinking is that 'X many gallons can handle Y number of fish'.
There are many things that affect the maximum bioload (rate of waste production) that an aquarium can handle. The two most important are volume and water change schedule. The volume basically sets a general range for the maximum amount of fish. The most important factor is the water change schedule.
The water change schedule dictates water quality. Without water changes tons of filtration on a 300 gallon tank can't handle even a school of tetras. Massive water changes on a smaller tank with adequate filtration can allow for a larger bioload than the average water change schedule on the same size tank.
There are mistakes being made on both sides when stocking is being discussed. The person simply asking 'How many fish can go in my 29' is assuming that stocking level is based just on volume, but they are not taking into account the water change schedule and other factors. More at fault are all of those who simply start answering the question without asking 'what is the water change schedule?' or even 'What filtration do you have?'. The person asking may simply not be advanced enough to realize they need to be aware of these other issues, but they are asking for help. The others are neglecting to fully address the issue at hand and allowing others to keep looking at stocking in the wrong way. Anyone who answers these types of questions without focusing mainly on water quality is not only unhelpful, but negligent. Answering without focusing on water quality harms the person asking the question by falsely affirming that stocking is based on volume.
Filtration does not just clean a certain volume of water the way the packaging suggests. Filters clean up after fish. A 75 with a school of full grown fancy goldfish needs a lot more filtration than if it housed a smaller bioload, like a few schools of tetras. Part of stocking an aquarium is providing the filtration to deal with those fish. Filtration can limit stocking. More filtration is never a bad thing. Barely meeting the minimal filtration needs of a certain setup is risking problems down the road. When the only or one of the filters stops running and you can't replace it for another week, if you only had the minimal to start with you are now faced with an under-filtered and overstocked tank.
In general the best measure of water quality is nitrate concentration. Nitrate is in general the end of the nitrogen cycle in an aquarium. In freshwater there are two main ways to remove nitrate, water changes and plants. With plants there has to be a very small bioload for the tank size and a lot of plants. This is rarely the case and even then small water changes are still needed. So in effect water changes are the only way to remove nitrate from the system. Nitrate itself will build in concentration over time. This can stress the fish, cause stunting, and can lead to illness and death. In addition, there are many other things that can build up over time. Growth inhibiting hormones are given off by many species of fish which can severely stunt fish of the same or closely related species and cause the same problems as nitrate. There are also dissolved organic compounds and other things that slowly build up over time. These also cause stress, illness, and even death. Even if extremes such as illness and death are not caused, a general failure to thrive can occur. This means that even though the fish seem fine, they should be doing even better (sometimes much better) if water quality was better. In general these chemicals will not build up if adequate water changes are done. Although we cannot test for these other harmful chemicals, they do tend to correlate well with nitrate concentration which is easily tested for. It is very important to keep the nitrate concentration under 20ppm. In cases of the tap water having a significant concentration the goal is then to keep the nitrate within 10ppm of the tap.
So a more appropriate answer to 'How many fish can I put in my X gallon tank?' without asking any questions would be: 'However many you can have with your water change schedule and keep the nitrate concentration under 20ppm'.
This brings us to another important aspect, the upper limits of aquariums. Even if you can keep the nitrate concentration down (high water quality), there is still an upper limit to the stocking density. I tried this with a 40high. I did massive weekly water changes and the water quality was in the ideal range. However, at a certain point it is simply so crowded that the fish are stressed not by water quality, but by the simple presence of so many other fish in the tank. This became apparent at about 80 fish in my situation in the 40high. At about that population mystery deaths started to occur as well as other signs that the fish were not thriving. I cut down the population and a happier, healthier community resulted. I do not suggest this and it is much better to prevent issues than treat them (don't fill your tank until they start dying to figure out the maximum population). Understocking is always better than risking overstocking.
In addition to simply being crowded, we come to the topic of big fish and their minimal tank sizes. Water quality is still most important, but with big fish you face a minimum in addition to that of maintaining water quality and space. In general, the tank should be at least as wide as the longest fish is long. So a cichlid that is 15" long should be in a tank at least 15" wide, to allow for an arguably comfortable turnaround. There are a few exceptions to this. Fish that are very long and extremely flexible do not require the turnaround width like bulkier fish do. These would be fish like ropefish and eels. These fish can be 18" long and turn around comfortably in a 12" wide tank. This does not mean you can crowd a bunch into a 55, but that the minimum width rule is not really in effect. Schooling fish also require larger than expected tanks. For example: giant danios generally only get to about 4" or so, but since they are so active and do like to school (a school is generally considered at least 6 fish) they should be in something like a 30long at minimum, some would even say a 55. Goldfish are another example. Many suggest 20 gallons for one and an additional ten gallons for each additional goldfish. This is a good guide, but since they do school this means that in order to have fancy goldfish you should really have at least a 75 gallon aquarium. For the more active, and larger, long-bodied goldfish it is generally recommended to have at least a 125 (or ideally a pond).
Fish behavior also needs to be taken into account. An active species needs more space than a less active species of the exact same size. So things like danios, although small, need a larger tank because they are so active and they school. Aggressive species need special attention as well. Although they can turnaround in a 125 and you may maintain the water quality, five oscars in a 125 will most likely end up killing each other, there is simply not enough room for all of them. This is especially true of cichlids when they pair off, they will become extremely aggressive and may lay claim to the entire tank as their territory. Many of them have territories in the wild larger than almost any aquarium they are kept in. So when they pair off they can turn what was once a relatively peaceful tank into a battlefield and even death camp.
Another commonly overlooked aspect is nocturnal fish. They hide away unseen during the day, but can be very active at night when the lights are out and the aquarist is not watching. As far as the keeper knows it is a medium fish that hides all the time and is very lazy. Only they do not realize that at night that fish is all over the place, possibly stressing out the diurnal fish that are now trying to rest. This can even cause things like mystery disappearances of fish. People see healthy fish and yet they keep disappearing. The culprit may be that otherwise very lazy catfish that at night goes after the sleeping fish the keeper has noticed are disappearing.
An increase in tank size does not mean the aquarist can now become lazy about water quality. Yes, a larger tank does mean that water quality will stay a little better, but that does not allow any laziness on the part of the aquarist. Since many upgrades are made for a growing fish, the water quality will still need to be maintained and water changes should remain just as often and as large.
Any aquarist needs to occasionally check the nitrate concentration to ensure that water quality is actually being maintained. Many have a tendency to get in the habit of a water change schedule but fail to adjust it to the fish growing. In addition, what seemed like a great water change schedule at first, may actually just mean that the nitrate concentration will rise much slower than usual, not actually maintain the ideal level of no more than 20ppm. If the tank is being under-maintained in other ways there could also be a significant increase in nitrate concentration. If debris is trapped in the gravel and not being removed it will eventually break down and be converted into nitrate. Many or probably most keepers tend to not maintain their filters often enough. This can cause the same problem. The debris they collected will eventually break down and cause nitrate problems. To help prevent these issues and catch them early it is important to check nitrate concentration on a regular basis (for example, once a month) even on tanks that are doing amazingly well. In addition the bioload of the tank increases over time simply due to the growth of the fish. A tank full of juvenile fish has a smaller bioload than the exact same stocking a year later. This increases the likelihood of the old water change schedule being inadequate.
Many aquarists feel it should 'not be fixed if it is not broken'. Unfortunately this can mean that problems that are slowly developing but not causing any major problems immediately are overlooked and develop to the point where they cannot be fixed and the result is harm to the fish, either by stunting, severe stress, or death. Assume you can improve things and work toward doing so.
For some reason many people believe that extra filtration helps keep the tank cleaner and therefore the tank doesn't need as many or as large of water changes. This is absolutely not the case. It is almost impossible to not have enough biofiltration (any cycled tank will have 0 ammonia and nitrite, therefore it has enough biofiltration). The other main function of filtration is to create healthy water flow and collect debris for you the aquarist to remove. So you can see the functions of filtration do not overlap with the functions of water changes. More filtration cannot compensate for inadequate water changes.
Stocking is far from a very over-simplified quick rule of thumb like 'one inch per gallon', 'one inch of fish per inch of tank length', and other 'rules'. It is very complex and everything from fish behavior to your ability to properly maintain water quality needs to be considered.