1.5" of fish per gallon of water

the inch per gallon "rule" basically sucks. it only applies to very small, tropical fish with a low bioload, like neon tetras.

like Mgamer said, you can't put a 10" oscar into a 10 gallon tank. you have to base tank size and fish stock more on the mass of the fish, the ammount of waste it produces, and how much swimming space it needs.

post what kind of fish you are interested in and your tank size, and we can help you choose ones that would work best for your tank.

the number and size of the fish you keep in your tank should be based on several factors. it should not be based solely on the chemistry of your water nor on the magic of the inch per gallon myth. it's more than simply a matter of successfully keeping the ammonia and nitrites at zero or the fact that you may have "great filtration". certainly these are important issues, but one of the most important factors is almost always overlooked. the number and size of fish that one maintains in a tank should be predicated on the "biology" and behaviour of those fish.

this means that the interraction of a mixed community tank and/or the behaviour of a single species is the single most important factor to consider. you want the fish you're keeping to have the ability to exhibit "normal" and functional behaviour. normal behaviour is important. abnormal behaviour results in stress. stress leads to disease. disease can lead to death. for example:

1. if species (A) does not get along with species (B), then these fish don't belong in the same tank no matter how few fish you have or how large the tank. "getting along" is based on the visual and behavioural Q's that each fish species is genetically programmed with. this is why African cichlids don't belong in the same tank with Central American cichlids for example. Africans don't "understand" Central American fish speak and this leads to behavioural problems and stress.

2. if species (A) has specific food or water chemistry requirements which are completely different from species (B), then these fish don't belong in the same tank no matter how large it is.

3. if species (A) is aggressive and defends a territory of 2 square feet, then you cannot expect to successfully keep more than one of these fish in a tank which is smaller than 2 square feet. convict cichlids are an excellent example of this ... this little fish aggressively defends territories of about two square feet give or take. if you have a tank which allows only that much room and no more, the convict will defend the entire tank to the detriment of any other fish (no matter how large it is) in that tank.

fish need room to swim without having the rest of the tanks inhabitants "in their face" continuously. when crowded, fish exhibit stress syndromes that result in poor color, improper fin form, insufficient metabolic development, do not exhibit proper musculature, do not develop properly functioning organ systems and most importantly slowly lose their inherant resistance to disease. this results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target..

think about your local lake or river --- the fish are free to inhabit whatever space suits them biologically. if it gets crowded by their standards, some will disperse and move to other areas where they again have the space they need to exhibit functional behaviour.

it's difficult to allow for that "space" in your tank -- the fish have no escape within the confines of your tank so it's up to you to insure that the fish have that space in the first place. if your tank "looks bare" .. it's probably just right.

Post pics of the "small algae eater" and the "betta (maybe)?", I or someone else here should be able to give you a positive ID.

no congos. they get close to 4" long, often more than that. they need room to swim. i'm going to push it with 6 congos and a ctenopoma acutirostre in a 45g with overfiltration and LOTS of live plants, and i know i'm going to be pushing it, so i'm not reccomending it for anyone else, lol. do as i say, not as i do.

The fish per gallon calculations are too simplified, but they are a good way for a store to tell someone that they shouldn't put 60 neons in a 10 gallon tank.

The ones based on surface area are a much better guide, if you're doing regular water changes. For instance, for a thousand square inches of surface area, one of the simpler fomulae would be:

inch
lenth #
of fish
1 100
1.5 67
2 50
2.5 40
3 33
4 25
5 20
6 17

So for a 20-gallon high tank, you'd have 288 square inches. So you could keep a MAXIMUM of 12 fish that would have a 2-1/2" adult length (not including tail). It's not perfect, but it's good for planning.

Your tank:
36x13" surface area
1 45
1.5 30
2 23
2.5 18
3 15
4 11
5 9
6 8

To my eye, 8 6-inch long fish in that tank would still be way too many, but not as bad as 10 of them! Some other formulae "roll off" more for larger fish.