Stocking an aquarium
There seems to be a lot od bad information and bad ways of thinking about stocking an aquarium.
Most common mistake and assumption: 'X many gallons can handle Y number of fish'.
There are MANY things that effect the maximum bioload (amount of fish plus food) 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 is water change schedule.
Really the water change schedule in general dictates water quality, which is really the most important aspect. 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, and 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.
Filtration does not just clean X many gallons 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 was heavily planted with a few low-waste producing fish. 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/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 that tank size and A LOT of plants. This is rarely the case and even then small water changes are 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 stunt conspecifics and cause the same problems as nitrate. There are also dissolved organic compounds and other things that slowly build up over time, causing stress, illness, and possibly 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 would be doing EVEN better (sometimes much better) if water quality was improved. In general these chemicals will not build up if adequate water changes are done. Although we cannot test for these other harmful chemcials, they do tend to correlate with nitrate concentration which is easily tested for. It is generally recommended to keep nitrate concentration under 20ppm.
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 and large fish. Even if you can keep the nitrate concentration down (water quality up), 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 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).
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 and schooling fish, you face a minimum in addition to that of maintaining water quality. 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, bichirs, 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.
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 one-because they are so active, and two-because 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 end up most likely 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. These fish were not made to be in aquariums. Many of them have territories in the wild larger than almost any aquarium they are kept in. So when they pair off they are not required to obey your feeling that there is enough room for everyone.
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 fishkeeper 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 seen disappearing.
An increase in tank size does not mean the keeper can now become lazy about water quality. Yes, a larger tank does mean that water quality will stay a little higher, but that does not allow any laziness on the part of the keeper. Since many upgrades are made for a growing fish, the water quality will still need to be maintained and water changes schould remain just as often and as large.
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