To answer this question, we will accept as a given that you have adequate filtration and aeration taking place. The number of fish a tank can hold is based on the filter's ability to maintain a clean environment faster than the fish can pollute it. You can have one inch of fish for every two square inches of tank area. For example, say you measure the top of your tank and find the dimensions are 8 inches by 20 inches. That's a total area of 160 square inches. Thus, you can safely keep 80 - 1 inch fish, or 40 - 2 inch fish, or 20 - 4 inch fish. The depth of the tank doesn't factor into this, because oxygen exchange only takes place at the water's surface. All tanks are at least 10 inches deep, so there's plenty of "swimming room". What we are concerned with is maintaining their environment.

The easiest answer to that question is "one fish at a time.'' As far as how many in total can safely survive, a frequently used rule-of-thumb is "up to a maximum of 1 inch of fish per gallon.'' Much discussion of this rule has suggested that it really should read "up to a maximum of 1 inch of SLIM-BODIED fish per gallon.'' "Slim-bodied'' could be fish such as neon tetras, White Cloud Mountain Minnows, danios etc.; "medium bodied'' might be red-tailed black sharks, tiger barbs, platys, cory cats etc.; "heavy bodied'' would be goldfish, oscars etc. In other words, this is only a rule of thumb, and the "maximum'' population that is safe and humane will vary from tank to tank. Factors that increase your possible fish load include: - regular and significant water changes, - HEALTHY live plants, and - more than one type of well-tended filtration (remember to think of your filter as alive; it needs care just as do your fish). Likewise, factors that decrease your possible load include: - erratic or sparse water changes, - no plants or UNhealthy live plants, and - limited or ill-tended filtration (an undergravel filter can do a great job, but if it fails for some reason and was the only filtration on the tank, a heavily stocked tank will experience much more disastrous consequences than one with a light load). So, back to adding fish. Often it is not practical to add fish one at a time - for instance, you find some especially great looking neons and want to add a small school (6 or 7 fish) to your recently cycled 20 gallon tank. You currently have one 2-inch pl*co and three 1.5-inch platys. Adding the neons will essentially double the "volume'' of fish in the tank. In this case, you will see the same effects as cycling your tank, i.e., an ammonia and nitrite spike before the bacteria grows to match the new fish population. Test your water frequently and be prepared to do emergency partial water changes if the ammonia levels go up too far. The bio-filter for your tank is only "fed'' by the wastes of the fish you have in the tank. This means that no matter how large your filter (e.g., one rated for a fully stocked 50 gal tank on your 20 gal), the bacteria population will be limited by the ``food'' it has. Few fish = small bacteria population. We are accustomed to thinking of bacteria reproduction as "explosive''. Many bacteria can double their population size in hours, after all... but as we have seen in the CYCLING SECTION, the appropriate nitrifying bacteria are relatively slow to reproduce. There will be a time delay between the increased waste production of additional fish, and increased waste processing by the bacteria. In extreme cases, the ammonia increase could harm or kill your fish before the bacteria population had time to "catch up'' to the amount of available nitrogenous wastes. This is why it is wise to add fish slowly and gradually. Safely bringing your tank's population up to the maximum load can take more than 6 months; in fact, it should be permitted to take at least that long. Leave breaking the rules to those with more years experience than they have fish.

Simple - to start with, just one or two small fish. For the first six weeks you have your tank it has to go through a process called "cycling". During that time your tank is building up a supply of good bacteria that help break down toxic fish waste into less harmful nitrates. After the inital six weeks, you can continue to add new fish slowly (about two every three weeks) until you reach the tanks limit. Now your limit is going to depend on what kind of fish you're keeping, and also on the kind of tank you have. Oh, and just a note, make sure you have a rectangular tank, hexagons or tanks that are taller than they are long can't hold very many fish at all! A good rule of thumb for tropical fish tanks is one inch of fish per gallon of water for small fish. For bigger fish like cichlids, goldfish (they grow huge), etc... you'll need at least 10 gallons per fish, with a minimum tank size of 20 gallons. In fact you probably can't raise cichlids in anything smaller than 30 gallons. In any case, it is always better to seem to have "too few" fish than to have too many.

Well really i think u should just read an online profile of the fish that u r goin to buy NEVER ! just go to the fish store and buy what ever fish that u think looks cool! This is because u can have more fish in ur tank if there personalities match! For example if u had a tank with many tetras(or any kind of peaceful fish) u can have more of them because they do not need the extra area to run and hide....but if u had for say a tank with some tetras and a tiger barb or balla shark u would need to make sure that all! fish have a place to hide and are not left out in the open to be attacked

Under stock your tank. Over filter your tank. Get a bigger tank.

Hello; You will find many answers out there on this question. In my breeding tanks (29-gallon) I frequently have up to 100 1-inch fish and probably as many babies. All of my tanks are established and all use undergravel filters UGF's. The difference is the effective filtering area of the filters in question. An undergravel filter has a total filtering area of the tank bottom plus the thickness of the gravel substrate. This cannot be matched by any filter I know of and is why I use them. Aquariums were so hard to maintain and the loses so great that very few people had aquariums until the undergravel filter was developed. Then the hobby expanded with leaps and bounds. The problem comes in with undergravel filters in their highly effective filtering ability. They break down waste fast and create a lot of nitrates --- exactly what a filter is supposed to do. Second, the water looks so clear and nice that most people do not think the gravel (filter media) needs cleaning and they let the system get too dirty. This substrate full of waste eventually slows the flow of oxygenated water and causes the aerobic bacteria ( needs oxygen to break down waste) to be slowly replaced with anerobic (does not need oxygen to break down waste) and polutes the system and smells. What could be simpler than using a siphon to vacuum the substrate? The cost is low as no filter media needs to be replaced and the water in the aquarium that is polluted with dissolved organic compounds is changed at the same time. On the bottom of a lake there is usually a layer of waste from fish, decaying matter, etc. The oxygen content of the water at the bottom of the lake is low, and since the there is very little water flow the waste is broken down slowly. In an aquarium the water flow is much higher and the oxygen content is also higher due to the exchange of water from the bottom to the top where it exchanges other gases C02 and hydrogen sulfate etc,. and picks up oxygen. Even the waste that is not sucked into the filter has water flow over it and breaks down waste faster than it would in the natural environment. Frequent cleaning of filter media removes waste before it can be converted into DOC's and nitrates. I have supplemental filters on most of my show tanks where I make my own filter media and add carbon, etc. I remove the filter media and clean it twice a week under tap water to remove the waste. I do not need the additional bacteria in the filter media as all my tanks use UGF's and the additional filters are only used for waste collection and chemical filtering. Enjoy! OFM

The 1 inch per gallon might work but for goldfish or other fish with a lot of mass at least 5 gallons per inch